[doc] Fix abc.update_abstractmethods markup (pythonGH-23576)

Add link to ABCMeta while at it.

Doc/library/abc.rst

@@ -336,14 +336,15 @@ The :mod:`abc` module also provides the following functions:
    .. versionadded:: 3.4
 
 .. function:: update_abstractmethods(cls)
+
    A function to recalculate an abstract class's abstraction status. This
    function should be called if a class's abstract methods have been
    implemented or changed after it was created. Usually, this function should
    be called from within a class decorator.
 
    Returns *cls*, to allow usage as a class decorator.
 
-   If *cls* is not an instance of ABCMeta, does nothing.
+   If *cls* is not an instance of :class:`ABCMeta`, does nothing.
 
    .. note::