fix(docs): remove extraneous dep from RN install script

[calebpollman]

Description of changes

Remove amazon-cognito-identity-js from React Native install script

Issue #, if available

Fixes #4946

Description of how you validated changes

Verified the update locally

Checklist

• Have read the Pull Request Guidelines
• PR description included

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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Latest commit: e70e241

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