docs: Fix keyboard example instructions (#1281)

examples/lib/stories/input/keyboard_example.dart

@@ -10,7 +10,7 @@ class KeyboardExample extends FlameGame with KeyboardEvents {
   static const String description = '''
     Example showcasing how to act on keyboard events.
     It also briefly showcases how to create a game without the FlameGame.
-    Usage: Use A S D F to steer Ember.
+    Usage: Use A S D W to steer Ember.
   ''';
 
   static final Paint white = BasicPalette.white.paint();