docs: Fix step 2 of space shooter tutorial delta values (#3200)

There was a small code Error in Step 2 of the Space Shooter Tutorial
```
the onPanUpdate method looked like this 
  void onPanUpdate(DragUpdateInfo info) {
    player.move(info.delta.global);
  }
```
but it should look like this
```
  void onPanUpdate(DragUpdateInfo info) {
    player.move(info.delta.global);
  }
```

doc/tutorials/space_shooter/step_2.md

@@ -67,7 +67,7 @@ class SpaceShooterGame extends FlameGame with PanDetector {
 
   @override
   void onPanUpdate(DragUpdateInfo info) {
-    player.move(info.delta.game);
+    player.move(info.delta.global);
   }
 }
 ```
@@ -116,7 +116,7 @@ class SpaceShooterGame extends FlameGame with PanDetector {
 
   @override
   void onPanUpdate(DragUpdateInfo info) {
-    player.move(info.delta.game);
+    player.move(info.delta.global);
   }
 }
 ```
@@ -181,7 +181,7 @@ class SpaceShooterGame extends FlameGame with PanDetector {
 
   @override
   void onPanUpdate(DragUpdateInfo info) {
-    player.move(info.delta.game);
+    player.move(info.delta.global);
   }
 }
 ```