Fix a test error from before

Author: ko2ic

packages/firebase_auth/test/firebase_auth_test.dart

@@ -400,7 +400,7 @@ void main() {
 
     test('updateEmail', () async {
       final String updatedEmail = 'atestemail@gmail.com';
-      auth.updateEmail(email: updatedEmail);
+      await auth.updateEmail(email: updatedEmail);
       expect(
         log,
         <Matcher>[