update install command for Mac OS (#16177)

when copying/pasting the command to install the Kubernetes on Mac OS using zsh, the command results in the following error,
`The specified key does not exist.`. This is usually because of the `\` that zsh automatically inserts into the command. The best practice is to use double-quotes and this is also the standard method according to https://google.github.io/styleguide/shell.xml?showone=Quoting#Quoting

content/en/docs/tasks/tools/install-kubectl.md

@@ -102,7 +102,7 @@ If you are on Ubuntu or another Linux distribution that support [snap](https://s
 1. Download the latest release:
 
     ```		 
-    curl -LO https://storage.googleapis.com/kubernetes-release/release/$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt)/bin/darwin/amd64/kubectl
+    curl -LO "https://storage.googleapis.com/kubernetes-release/release/$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt)/bin/darwin/amd64/kubectl"
     ```
 
     To download a specific version, replace the `$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt)` portion of the command with the specific version.