gh-101100: Fix Sphinx nitpick in unittest.mock.rst

Removes from `.nitignore`. GH-124106 fixed most the warnings but left
this one unfixed. GH-113163 fixed many of the dunders referring to the
general definitions but this case refers to implementing a mock. Match
what `__eq__` and `__ne__` did a few lines before to refer to an
implementing instance.

Author: cmaloney

Doc/library/unittest.mock.rst

@@ -2223,7 +2223,7 @@ return something else::
    >>> mock == 3
    True
 
-The return value of :meth:`MagicMock.__iter__` can be any iterable object and isn't
+The return value of :meth:`!__iter__` can be any iterable object and isn't
 required to be an iterator:
 
    >>> mock = MagicMock()

Doc/tools/.nitignore

@@ -29,7 +29,6 @@ Doc/library/test.rst
 Doc/library/tkinter.rst
 Doc/library/tkinter.scrolledtext.rst
 Doc/library/tkinter.ttk.rst
-Doc/library/unittest.mock.rst
 Doc/library/urllib.parse.rst
 Doc/library/urllib.request.rst
 Doc/library/wsgiref.rst