fix(firebase_core): broken homepage link in pubspec.yaml (#9314)

Author: OutdatedGuy

packages/firebase_core/firebase_core/pubspec.yaml

@@ -1,7 +1,7 @@
 name: firebase_core
 description: Flutter plugin for Firebase Core, enabling connecting to multiple
   Firebase apps.
-homepage: https://firebase.flutter.dev/docs/core/usage
+homepage: https://firebase.flutter.dev/docs/core/usage/
 repository: https://github.com/firebase/flutterfire/tree/master/packages/firebase_core/firebase_core
 version: 1.20.0