docs: Added superkey to example widget (#326)

Author: KarthiDreamr

packages/firebase_ui_auth/README.md

@@ -38,6 +38,8 @@ import 'package:firebase_auth/firebase_auth.dart' hide EmailAuthProvider;
 import 'package:firebase_ui_auth/firebase_ui_auth.dart';
 
 class MyApp extends StatelessWidget {
+  const MyApp({super.key});
+
   @override
   Widget build(BuildContext context) {
     final providers = [EmailAuthProvider()];