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

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

----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/master/INTEGRATION_TESTS.md)?
	* [ ] Did you use `cdk-integ` to deploy the infrastructure and generate the snapshot (i.e. `cdk-integ` without `--dry-run`)?

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: yamatatsu

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)