gh-155631: Remove xml.dom.rst from the nit-picky mode exceptions

It no longer produces warnings in the Sphinx nit-picky mode.

Author: serhiy-storchaka

Doc/tools/.nitignore

@@ -24,7 +24,6 @@ Doc/library/urllib.request.rst
 Doc/library/wsgiref.rst
 Doc/library/xml.dom.minidom.rst
 Doc/library/xml.dom.pulldom.rst
-Doc/library/xml.dom.rst
 Doc/library/xml.sax.reader.rst
 Doc/library/xml.sax.rst
 Doc/library/xmlrpc.client.rst