Fixes grammatical error in 'Creating the Enemy'

The existing text reads "Select the Mob node and set it's Gravity Scale..."

It is instead grammatically correct to say "and set its" without an apostrophe.

getting_started/first_2d_game/04.creating_the_enemy.rst

@@ -24,7 +24,7 @@ Click Scene -> New Scene from the top menu and add the following nodes:
 Don't forget to set the children so they can't be selected, like you did with
 the Player scene.
 
-Select the ``Mob`` node and set it's ``Gravity Scale``
+Select the ``Mob`` node and set its ``Gravity Scale``
 property in the :ref:`RigidBody2D <class_RigidBody2D>`
 section of the inspector to ``0``.
 This will prevent the mob from falling downwards.