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.
diff --git a/Doc/library/markup.rst b/Doc/library/markup.rst
index 49794ef..1b4cca5 100644
--- a/Doc/library/markup.rst
+++ b/Doc/library/markup.rst
@@ -23,7 +23,7 @@
    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 @@
    xml.sax.handler.rst
    xml.sax.utils.rst
    xml.sax.reader.rst
-   xml.etree.elementtree.rst
+   pyexpat.rst