mrhlpr: Reword comment request (MR 38)

While I understand the intent behind the original message, I always end
up reading it as that providing a thank you comment is discouraged.
While I think the intended meaning is obvious if you think about it, to
me writing it this way feels like a better way to communicate it.

mrhlpr/frontend.py

@@ -270,6 +270,6 @@ def main():
         # TODO: Check that everything passed
         comment = args.comment
         if not comment:
-            comment = input("Provide a thank you comment (discouraged, but can be empty): ")
+            comment = input("Provide a thank you comment (can be empty — discouraged): ")
         mr.finish(mr_id, comment, args.no_cache, args.skip_approve)
         print_status(mr_id)