docs: Add events import in space shooter tutorial step_2 (#3231)

DragUpdateInfo is included in the events package but is not in the docs.


When following the shooter tutorial I had issues cause the
DragUpdateInfo was undefined. upon further investigation I found that it
was in the event import instead

Co-authored-by: Lukas Klingsbo <me@lukas.fyi>

doc/tutorials/space_shooter/step_2.md

@@ -12,6 +12,7 @@ use the `onPanUpdate` method. The updated code will look like the following:
 
 ```dart
 import 'package:flame/input.dart';
+import 'package:flame/events.dart';
 
 class SpaceShooterGame extends FlameGame with PanDetector {
   late Player player;