typo fix GoogleSignIn.cs

[VitalikBerashvili]

Pull Request Title

Fix Typo in GoogleSignIn.cs

Description

This pull request fixes a typo in the GoogleSignIn.cs file.

Changes Made:

• File Modified: GoogleSignIn.cs
• Summary of Changes:
  • Corrected the typo in the authenication field to authentication in the comments section.

Checklist

• Corrected the typo in the documentation.
• Verified the correctness of the comment.

Additional Notes

This change ensures the documentation is accurate and free of spelling errors.

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Allow edits by maintainers: [x]

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[jonny-jeahyunchoi]

Since external library code is included in the repository, it may be better not to correct typos for version control purposes.
https://github.com/googlesamples/google-signin-unity