bpo-28556: Remove another mention of metaclass of Generic in typing docs (pythonGH-16743)

Metaclass was removed in Python 3.7 (there is already a `versionchanged` item about this).

https://bugs.python.org/issue28556
(cherry picked from commit 8144095)

Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>

Author: miss-islington

Doc/library/typing.rst

@@ -230,8 +230,8 @@ A user-defined class can be defined as a generic class.
 single type parameter ``T`` . This also makes ``T`` valid as a type within the
 class body.
 
-The :class:`Generic` base class uses a metaclass that defines
-:meth:`__getitem__` so that ``LoggedVar[t]`` is valid as a type::
+The :class:`Generic` base class defines :meth:`__class_getitem__` so that
+``LoggedVar[t]`` is valid as a type::
 
    from typing import Iterable