S04: fix incorrect bonus XP value

("no time for reinforcements" achievement reward)

Author: inferno8

scenarios/04_Southern_Nations.cfg

@@ -1348,7 +1348,7 @@
                         [filter]
                             id=Mehir
                         [/filter]
-                        experience="$($this_unit.experience + 100)"
+                        experience="$($this_unit.experience + 50)"
                     [/modify_unit]
                     [store_unit]
                         [filter]