clarified "App" syntax

This class would overlap with the default xamarin forms `App` class name.

Firebase.CloudMessaging/component/GettingStarted.md

@@ -91,7 +91,7 @@ Once you have your `GoogleService-Info.plist` file downloaded in your computer,
 4. Add the following line of code somewhere in your app, typically in your AppDelegate's `FinishedLaunching` method (don't forget to import `Firebase.Core` namespace):
 
 ```csharp
-App.Configure ();
+Firebase.Core.App.Configure();
 ```
 
 ### Register for remote notifications
@@ -594,4 +594,4 @@ You can send notification messages to iOS and Android devices using the Notifica
 [24]: https://firebase.google.com/docs/cloud-messaging/send-with-console
 [note_icon]: https://cdn3.iconfinder.com/data/icons/UltimateGnome/22x22/apps/gnome-app-install-star.png
 [advice_icon]: https://cdn1.iconfinder.com/data/icons/nuove/22x22/actions/messagebox_warning.png
-[warning_icon]: https://cdn2.iconfinder.com/data/icons/freecns-cumulus/32/519791-101_Warning-20.png
+[warning_icon]: https://cdn2.iconfinder.com/data/icons/freecns-cumulus/32/519791-101_Warning-20.png