gh-92082: contextlib docs: Change aclosing from a class to a function for consistency (GH-92155)

Signed-off-by: prwatson <prwatson@redhat.com>
(cherry picked from commit 958f21c5)

Co-authored-by: Thaddeus1499 <104600742+Thaddeus1499@users.noreply.github.com>

Doc/library/contextlib.rst

@@ -181,7 +181,7 @@ Functions and classes provided:
    ``page.close()`` will be called when the :keyword:`with` block is exited.
 
 
-.. class:: aclosing(thing)
+.. function:: aclosing(thing)
 
    Return an async context manager that calls the ``aclose()`` method of *thing*
    upon completion of the block.  This is basically equivalent to::