fix: flutterfire_ui README links

[pedrobalao]

Description

The flutterfire_ui README file had some links broken, as they were pointing to a 'docs' folder instead of 'doc'. This PR intends to fix it.

Related Issues

#8628

Checklist

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This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• [*] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [NA] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• [NA] All existing and new tests are passing.
• [NA] I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• Yes, this is a breaking change.
• [*] No, this is not a breaking change.

[kevinthecheung]

One more on line 58 ("Learn more in the Integrating your first screen section...").

[kevinthecheung]

Thanks!