📚[ Firebase_messaging] gettings started docs have buggy code

[Tejaswgupta]

The getting started of firebase_messaging plugin for the android section asks to create an Application.java file and put
GeneratedPluginRegistrant.registerWith(registry);

Where it throws and error and as discussed in an issue for the same , it should have been
FirebaseMessagingPlugin.registerWith(registry.registrarFor("io.flutter.plugins.firebasemessaging.FirebaseMessagingPlugin"));

The docs have not been update yet and can cause issue to someone who is getting started with the plugin

[dackers86]

PR linked at #3963

[dackers86]

This is due to be refactored as part of the messaging rework. Please continue to use the code above or the example PR for now