Apply suggestions from code review

Co-authored-by: Allison Ryan <77211884+allisonryan0002@users.noreply.github.com>

lib/game/game_assets.dart

@@ -1,8 +1,8 @@
 import 'package:pinball/game/game.dart';
 
-/// Add methods to help loading and caching game assets
+/// Add methods to help loading and caching game assets.
 extension PinballGameAssetsX on PinballGame {
-  /// Pre load the initial assets of the game
+  /// Pre load the initial assets of the game.
   Future<void> preLoadAssets() async {
     await Future.wait([
       images.load(Ball.spritePath),