Fix dartfmt issue.

Author: slightfoot

packages/firebase_auth/example/lib/main.dart

@@ -99,7 +99,7 @@ class _MyHomePageState extends State<MyHomePage> {
         (AuthException authException) {
       setState(() {
         _message = Future<String>.value(
-            'Phone numbber verification failed. Code: ${authException.code}. Message: ${authException.message}');
+            'Phone number verification failed. Code: ${authException.code}. Message: ${authException.message}');
       });
     };