[LoW] S3: fix mismatched coords of Urudin retreat (#7221)

The trigger for the move_to is now an area of 19 hexes.

Co-authored-by: Wedge009 <wedge009@wedge009.net>
(cherry picked from commit 0b483953e9)

data/campaigns/Legend_of_Wesmere/scenarios/chapter1/03_Kalian_under_Attack.cfg

@@ -188,6 +188,7 @@
         [ai]
             [engine]
                 name=lua
+                # Note that the move_full co-ordinates should match the moveto event for Urudin below
                 code=<<
                 local my_ai = { }
 
@@ -197,7 +198,7 @@
                         local mhp, hp = urudin.max_hitpoints, urudin.hitpoints
                         local turn = wesnoth.current.turn
                         if turn >= 3 or hp < mhp / 2 then
-                            ai.move_full(urudin, 20, 6)
+                            ai.move_full(urudin, 12, 4)
                         end
                     end
                 end
@@ -451,9 +452,13 @@
     [event]
         name=moveto
         [filter]
-            x=12
-            y=4
             id=Urudin
+            [filter_location]
+                # Note that these co-ordinates should match the AI code for Urudin's side declared above
+                x=12
+                y=4
+                radius=3
+            [/filter_location]
         [/filter]
         [if]
             [variable]