docs(auth): update the incorrect "getting started" link (#11440)

Author: DavidLemayian

packages/firebase_auth/firebase_auth/README.md

@@ -9,7 +9,7 @@ To learn more about Firebase Auth, please visit the [Firebase website](https://f
 
 ## Getting Started
 
-To get started with Firebase Auth for Flutter, please [see the documentation](https://firebase.flutter.dev/docs/auth/overview).
+To get started with Firebase Auth for Flutter, please [see the documentation](https://firebase.google.com/docs/auth/flutter/start).
 
 ## Usage