Mention required steps for Godot 3.2+ in part 3 of FPS tutorial (#4510)

Explanation from TwistedTwigleg:

Something changed with GridMaps since Godot 3.0 and that caused the GridMap nodes to lose their reference to the library of GridMap tiles, which makes 98% of the level missing! Reassigning the GridMap libraries should be all that is required and once assigned the level should function like expected.

Co-authored-by: Hugo Locurcio <[email protected]>

tutorials/3d/fps_tutorial/part_three.rst

@@ -33,6 +33,18 @@ or :kbd:`F6` on keyboard, and give each a try.
 .. warning:: ``Space_Level.tscn`` is more graphically demanding of the GPU than ``Ruins_Level.tscn``. If your computer is struggling to render
           ``Space_Level.tscn``, try using ``Ruins_Level.tscn`` instead.
 
+.. note::
+
+    Due to Godot updates since this tutorial was published, if you are using Godot 3.2 or later, you may need to apply the following changes to the Space Level and Ruins Level scenes:
+
+    - Open ``res://assets/Space_Level_Objects/Space_Level.tscn``.
+    - In the Scene tree dock, select the **Floor_and_Celing** node. In the Inspector dock, if the Mesh Library field under GridMap is ``[empty]``, set it to ``Space_Level_Mesh_Lib.tres`` by dragging the file ``res://assets/Space_Level_Objects/Space_Level_Mesh_Lib.tres`` from the FileSystem dock to that field.
+    - Do the same for the **Walls** node.
+    
+    - Open ``res://assets/Ruin_Level_Objects/Ruins_Level.tscn``.
+    - In the Scene tree dock, select the **Floor** node. In the Inspector dock, if the Mesh Library field under GridMap is ``[empty]``, set it to ``Ruin_Level_Mesh_Lib.tres`` by dragging the file ``res://assets/Ruin_Level_Objects/Ruin_Level_Mesh_Lib.tres`` from the FileSystem dock into that field.
+    - Do the same for the **Walls** node.
+
 You might have noticed there are several :ref:`RigidBody <class_RigidBody>` nodes placed throughout the level.
 We can place ``RigidBody_hit_test.gd`` on them and then they will react to being hit with bullets, so let's do that!