Move xml.etree higher and xml.parsers.expat lower in the markup ToC.

I assume that most users looking at this page are looking for a library
to process XML, not a low-level parser.  First proposed in #11379.

Doc/library/markup.rst

@@ -23,7 +23,7 @@ definition of the Python bindings for the DOM and SAX interfaces.
    html.rst
    html.parser.rst
    html.entities.rst
-   pyexpat.rst
+   xml.etree.elementtree.rst
    xml.dom.rst
    xml.dom.minidom.rst
    xml.dom.pulldom.rst
@@ -31,4 +31,4 @@ definition of the Python bindings for the DOM and SAX interfaces.
    xml.sax.handler.rst
    xml.sax.utils.rst
    xml.sax.reader.rst
-   xml.etree.elementtree.rst
+   pyexpat.rst