docs(auth): update phone-auth verifyPhoneNumber link

[rohitsx]

Description

This PR updates a broken or outdated documentation link in the Phone Auth guide.
The existing link for verifyPhoneNumber pointed to the old phoneauthprovider path.
It now correctly links to the consolidated Auth reference section:
https://rnfirebase.io/reference/auth#verifyPhoneNumber.

Related issues

None — this is a minor documentation fix.

Release Summary

docs(auth): update verifyPhoneNumber link in phone-auth documentation

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
  • Other (macOS, web)
• My change includes tests;
  • e2e tests added or updated in packages/**/e2e
  • jest tests added or updated in packages/**/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

Documentation-only change.
Verified that the new link correctly opens the verifyPhoneNumber section in the Auth reference.

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