Add missing changelog entry

CHANGELOG.md

@@ -235,6 +235,7 @@
     Bug #6043: Actor can have torch missing when torch animation is played
     Bug #6047: Mouse bindings can be triggered during save loading
     Bug #6136: Game freezes when NPCs try to open doors that are about to be closed
+    Bug #6142: Groundcover plugins change cells flags
     Bug #6276: Deleted groundcover instances are not deleted in game
     Bug #6294: Game crashes with empty pathgrid
     Feature #390: 3rd person look "over the shoulder"