Fix README.md snippet
Fix README.md snippet

Fix Register a user example code snippet in README.md
firebase/flutterfire,firebase/flutterfire,firebase/flutterfire,firebase/flutterfire,googleads/googleads-mobile-flutter,firebase/flutterfire,googleads/googleads-mobile-flutter,firebase/flutterfire,firebase/flutterfire,firebase/flutterfire,firebase/flutterfire,googleads/googleads-mobile-flutter,firebase/flutterfire,googleads/googleads-mobile-flutter