Fix typo (#1927)

first we describe ``process_input`` and then ``process_movement``

tutorials/3d/fps_tutorial/part_one.rst

@@ -312,7 +312,7 @@ so it can move through the game world.
 
 _________
 
-Let's look is ``process_movement`` next:
+Let's look at ``process_input`` next:
 
 First we set ``dir`` to an empty :ref:`Vector3 <class_Vector3>`.