Fix template app documentation (#131125)

The PR fixes template app documentation

Author: DavidIsa

packages/flutter_tools/templates/app/lib/main.dart.tmpl

@@ -50,7 +50,7 @@ class MyApp extends StatelessWidget {
         // This is the theme of your application.
         //
         // TRY THIS: Try running your application with "flutter run". You'll see
-        // the application has a blue toolbar. Then, without quitting the app,
+        // the application has a purple toolbar. Then, without quitting the app,
         // try changing the seedColor in the colorScheme below to Colors.green
         // and then invoke "hot reload" (save your changes or press the "hot
         // reload" button in a Flutter-supported IDE, or press "r" if you used