Change name of exported variable

Since the exported `PackedScene` variable is named `mob_scene`, the generated script variable is called `Mob Scene`, not `Mob`.

getting_started/first_2d_game/05.the_main_game_scene.rst

@@ -104,7 +104,7 @@ Add a script to ``Main``. At the top of the script, we use ``export
         }
     }
 
-Click the ``Main`` node and you will see the ``Mob`` property in the Inspector
+Click the ``Main`` node and you will see the ``Mob Scene`` property in the Inspector
 under "Script Variables".
 
 You can assign this property's value in two ways: