docs(firebase_ui_auth): use a valid BuildContext in onSignedIn() to navigate correctly

[EchoEllet]

Description

The current code snippet in the README of firebase_ui_auth does not function after logging in with any authentication provider and will produce a runtime error.

This is due to passing the BuildContext of the root app widget as an argument to Navigator.pushReplacementNamed(), which lacks the details required to navigate to the profile screen, which is only defined in a BuildContext that is under MaterialApp.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• All unit tests pass (melos run test:unit:all doesn't fail).
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.