Correct class name from PullDom to PullDOM

https://github.com/python/cpython/blob/9420795b47ac88f31315a8d1041e2c66c2cd9a8b/Lib/xml/dom/pulldom.py#L13

Author: guoci

Doc/library/xml.dom.pulldom.rst

@@ -74,7 +74,7 @@ given point) or to make use of the :func:`DOMEventStream.expandNode` method
 and switch to DOM-related processing.
 
 
-.. class:: PullDom(documentFactory=None)
+.. class:: PullDOM(documentFactory=None)
 
    Subclass of :class:`xml.sax.handler.ContentHandler`.