Fix missing var in NavigationAgent script example

Fixes missing var for CharacterBody3D NavigationAgent script example.

tutorials/navigation/navigation_using_navigationagents.rst

@@ -187,6 +187,7 @@ This script adds basic navigation movement to a CharacterBody3D with a Navigatio
 
     @export var movement_speed : float = 4.0
     @onready var navigation_agent : NavigationAgent3D = get_node("NavigationAgent3D")
+    var movement_delta : float
 
     func set_movement_target(movement_target : Vector3):
         navigation_agent.set_target_location(movement_target)