fix command error message. There are really only two arguments supported (the…

fix command error message. There are really only two arguments supported (the optional 6 had been removed, but I forgot to change the message

cms/djangoapps/contentstore/management/commands/clone.py

@@ -20,7 +20,7 @@ class Command(BaseCommand):
 
     def handle(self, *args, **options):
         if len(args) != 2:
-            raise CommandError("clone requires either two or six arguments: <source-location> <dest-location>")
+            raise CommandError("clone requires either two arguments: <source-location> <dest-location>")
 
         source_location_str = args[0]
         dest_location_str = args[1]