EI S11: Fixed missing prisoners (fixes #3237)

The problem was introduced by commit 0f511e8 due to the [modify_unit] tag apparently causing prisoners to disappear because technically they were duplicates as they had merely been unstored and still existed on the recall list as well.

data/campaigns/Eastern_Invasion/scenarios/11_Captured.cfg

@@ -317,11 +317,11 @@
                     [/variable]
 
                     [then]
-                        [unstore_unit]
-                            variable=possible_prisoners[0]
+                        [recall]
+                            find_in=possible_prisoners[0]
                             x,y=$this_item.x,$this_item.y
-                            find_vacant=yes
-                        [/unstore_unit]
+                            show=no
+                        [/recall]
 
                         {CLEAR_VARIABLE possible_prisoners[0]}
                     [/then]