Update _decorators.py

Update docstring

Author: alexitkes

pandas/util/_decorators.py

@@ -224,7 +224,7 @@ def deprecate_nonkeyword_arguments(
         The version in which positional arguments will become
         keyword-only.
 
-    allowed_args : list, int or None, default=None
+    allowed_args : list or int, optional
         In case of list, it must be the list of names of some
         first arguments of the decorated functions that are
         OK to be given as positional arguments. In case of an