chore(cognito): fix grammar in UserPool IDP docstrings

[douglasnaphas]

Closes gh-29254.

Issue # (if applicable)

Closes #29254.

Reason for this change

This fixes minor grammar issues in docs.

Description of changes

I made no functional code changes, only changes to docs via comments. I made no design decisions.

Description of how you validated changes

Doc changes only.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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

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• Commit ID: 275080e
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Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).