Update instructions for collision mask in Your first 2D game (#5485)

getting_started/first_2d_game/04.creating_the_enemy.rst

@@ -26,7 +26,7 @@ the Player scene.
 
 In the :ref:`RigidBody2D <class_RigidBody2D>` properties, set ``Gravity Scale``
 to ``0``, so the mob will not fall downward. In addition, under the
-``PhysicsBody2D`` section, click the ``Mask`` property and uncheck the first
+:ref:`CollisionObject2D <class_CollisionObject2D>` section, click the ``Mask`` property and uncheck the first
 box. This will ensure the mobs do not collide with each other.
 
 .. image:: img/set_collision_mask.png