docs(firebase_messaging): simplify notifications example (#6662)
docs(firebase_messaging): simplify notifications example (#6662)

Co-authored-by: Russell Wheatley <75fba59efea7a8df4f4e919c414100a3bad6113b@gmail.com>
firebase/flutterfire,firebase/flutterfire,firebase/flutterfire,firebase/flutterfire,firebase/flutterfire,firebase/flutterfire,firebase/flutterfire,firebase/flutterfire,firebase/flutterfire,firebase/flutterfire