Fix example project

Author: olexale

example/lib/main.dart

@@ -45,7 +45,7 @@ class _MyHomePageState extends State<MyHomePage> {
             ),
             Text(
               '$_counter',
-              style: Theme.of(context).textTheme.headline4,
+              style: Theme.of(context).textTheme.headlineMedium,
             ),
           ],
         ),