downgrad mapCopyright lint to suggestion

(as per today's discussion with tabascoeye, mapCopyright should not be
required, though I've left it as recommended)

lib/Properties.hs

@@ -73,7 +73,7 @@ checkMap = do
     $ complain "The map's tile size must be 32 by 32 pixels."
 
   unlessHasProperty "mapCopyright"
-    $ complain "must give the map's copyright via the \"mapCopyright\" property."
+    $ suggest "document the map's copyright via the \"mapCopyright\" property."
 
   -- TODO: this doesn't catch collisions with the default start layer!
   whenLayerCollisions (\(Property name _) -> name == "exitUrl" || name == "startLayer")