chore(cognito-identitypool): update README to encourage least privilege

[Leo10Gama]

Issue # (if applicable)

N/A

Reason for this change

To ensure customers are using the module securely, we want to encourage them to follow the best practice of the principal of least privilege. That is, customers are only providing as much access as necessary to complete the task, and no privileges greater than that.

Description of changes

• Updated the README to include sentences around the privilege and access granting sections encouraging only giving the least amount of privileges necessary.
• Updated the README example to remove wildcard privilege.

Description of how you validated changes

yarn rosetta:extract passes.

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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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).

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 207d882
• Result: SUCCEEDED
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[mergify]

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).

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