develop: update env file code examples

develop/dev-guide-sample-application-nodejs-prisma.md

@@ -101,7 +101,7 @@ Connect to your TiDB cluster depending on the TiDB deployment option you've sele
 6. Edit the `.env` file, set up the environment variable `DATABASE_URL` as follows, and replace the corresponding placeholders `{}` with the connection string in the connection dialog:
 
     ```dotenv
-    DATABASE_URL={connection_string}
+    DATABASE_URL='{connection_string}'
     ```
 
     > **Note**
@@ -138,7 +138,7 @@ Connect to your TiDB cluster depending on the TiDB deployment option you've sele
 5. Edit the `.env` file, set up the environment variable `DATABASE_URL` as follows, replace the corresponding placeholders `{}` with connection parameters on the connection dialog:
 
     ```dotenv
-    DATABASE_URL=mysql://{user}:{password}@{host}:4000/test?sslaccept=strict&sslcert={downloaded_ssl_ca_path}
+    DATABASE_URL='mysql://{user}:{password}@{host}:4000/test?sslaccept=strict&sslcert={downloaded_ssl_ca_path}'
     ```
 
     > **Note**
@@ -167,7 +167,7 @@ Connect to your TiDB cluster depending on the TiDB deployment option you've sele
 2. Edit the `.env` file, set up the environment variable `DATABASE_URL` as follows, replace the corresponding placeholders `{}` with connection parameters of your TiDB cluster:
 
     ```dotenv
-    DATABASE_URL=mysql://{user}:{password}@{host}:4000/test
+    DATABASE_URL='mysql://{user}:{password}@{host}:4000/test'
     ```
 
    If you are running TiDB locally, the default host address is `127.0.0.1`, and the password is empty.

develop/dev-guide-sample-application-ruby-rails.md

@@ -97,7 +97,7 @@ Connect to your TiDB cluster depending on the TiDB deployment option you've sele
 6. Edit the `.env` file, set up the `DATABASE_URL` environment variable as follows, and copy the connection string from the connection dialog as the variable value.
 
     ```dotenv
-    DATABASE_URL=mysql2://{user}:{password}@{host}:{port}/{database_name}?ssl_mode=verify_identity
+    DATABASE_URL='mysql2://{user}:{password}@{host}:{port}/{database_name}?ssl_mode=verify_identity'
     ```
 
    > **Note**
@@ -126,7 +126,7 @@ Connect to your TiDB cluster depending on the TiDB deployment option you've sele
 5. Edit the `.env` file, set up the `DATABASE_URL` environment variable as follows, copy the connection string from the connection dialog as the variable value, and set the `sslca` query parameter to the file path of the CA certificate downloaded from the connection dialog:
 
     ```dotenv
-    DATABASE_URL=mysql2://{user}:{password}@{host}:{port}/{database}?ssl_mode=verify_identity&sslca=/path/to/ca.pem
+    DATABASE_URL='mysql2://{user}:{password}@{host}:{port}/{database}?ssl_mode=verify_identity&sslca=/path/to/ca.pem'
     ```
 
    > **Note**
@@ -149,7 +149,7 @@ Connect to your TiDB cluster depending on the TiDB deployment option you've sele
 2. Edit the `.env` file, set up the `DATABASE_URL` environment variable as follows, and replace the `{user}`, `{password}`, `{host}`, `{port}`, and `{database}` with your own TiDB connection information:
 
     ```dotenv
-    DATABASE_URL=mysql2://{user}:{password}@{host}:{port}/{database}
+    DATABASE_URL='mysql2://{user}:{password}@{host}:{port}/{database}'
     ```
 
    If you are running TiDB locally, the default host address is `127.0.0.1`, and the password is empty.

tidb-cloud/serverless-driver-kysely-example.md

@@ -82,7 +82,7 @@ To complete this tutorial, you need the following:
 2. Set the environment variable `DATABASE_URL` in your local environment. For example, in Linux or macOS, you can run the following command:
 
     ```bash
-    export DATABASE_URL=mysql://[username]:[password]@[host]/[database]
+    export DATABASE_URL='mysql://[username]:[password]@[host]/[database]'
     ```
 
 ### Step 3. Use Kysely to query data
@@ -275,7 +275,7 @@ mysql://[username]:[password]@[host]/[database]
 3. Test your code locally:
 
    ```
-   export DATABASE_URL=mysql://[username]:[password]@[host]/[database]
+   export DATABASE_URL='mysql://[username]:[password]@[host]/[database]'
    next dev
    ```
 
@@ -286,7 +286,7 @@ mysql://[username]:[password]@[host]/[database]
 1. Deploy your code to Vercel with the `DATABASE_URL` environment variable:
 
    ```
-   vercel -e DATABASE_URL=mysql://[username]:[password]@[host]/[database] --prod
+   vercel -e DATABASE_URL='mysql://[username]:[password]@[host]/[database]' --prod
    ```
 
     After the deployment is complete, you will get the URL of your project. 

tidb-cloud/serverless-driver-prisma-example.md

@@ -68,7 +68,7 @@ To complete this tutorial, you need the following:
 2. In the root directory of your project, create a file named `.env`, define an environment variable named `DATABASE_URL` as follows, and then replace the placeholders `[]` in this variable with the corresponding parameters in the connection string.
 
     ```dotenv
-    DATABASE_URL="mysql://[username]:[password]@[host]:4000/[database]?sslaccept=strict"
+    DATABASE_URL='mysql://[username]:[password]@[host]:4000/[database]?sslaccept=strict'
     ```
 
    > **Note:**