chore(ssm): fix removed API ParameterStoreString (#19585)

This PR replace `ParameterStoreString` to `StringParameter.fromStringParameterAttributes` in README.
`ParameterStoreString` has been removed.

----

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packages/@aws-cdk/aws-ssm/README.md

@@ -30,7 +30,7 @@ import * as ssm from '@aws-cdk/aws-ssm';
 ## Using existing SSM Parameters in your CDK app
 
 You can reference existing SSM Parameter Store values that you want to use in
-your CDK app by using `ssm.ParameterStoreString`:
+your CDK app by using `ssm.StringParameter.fromStringParameterAttributes`:
 
 [using SSM parameter](test/integ.parameter-store-string.lit.ts)