fix: Clarify SpriteGroupComponent.updateSprite assertion (#3317)

Now the assertion is slightly clearer for what the user has done wrong
when calling the `SpriteGroupComponent.updateSprite` in an instance
without an initialized `sprites` map.

packages/flame/lib/src/components/sprite_group_component.dart

@@ -100,7 +100,11 @@ class SpriteGroupComponent<T> extends PositionComponent
 
   /// Updates the sprite for the given key.
   void updateSprite(T key, Sprite sprite) {
-    assert(_sprites != null, 'Sprites map is not yet initialized');
+    assert(
+      _sprites != null,
+      'This call can only be made after the sprites map has been initialized, '
+      'which should be done in either the constructor or in onLoad.',
+    );
     _sprites?[key] = sprite;
     _resizeToSprite();
   }