enable Material 3 on jsonexample (#1620)

jsonexample/lib/main.dart

@@ -33,7 +33,8 @@ class MyApp extends StatelessWidget {
     return MaterialApp(
       title: 'Flutter Demo',
       theme: ThemeData(
-        primarySwatch: Colors.blue,
+        colorSchemeSeed: Colors.blue,
+        useMaterial3: true,
       ),
       home: const MyHomePage(),
     );