[commands] Fix erroneous string in dm_only check

discord/ext/commands/core.py

@@ -1444,7 +1444,7 @@ def dm_only():
 
     def predicate(ctx):
         if ctx.guild is not None:
-            raise PrivateMessageOnly('This command cannot be used in private messages.')
+            raise PrivateMessageOnly('This command can only be used in private messages.')
         return True
 
     return check(predicate)