docs: Move the "Network Filtering" page one level up in the hierarchy

"Network Filtering" is not directly related to "Networks".

Suggested by Daniel P. Berrange.

docs/sitemap.html.in

@@ -111,12 +111,10 @@
               <li>
                 <a href="formatnetwork.html">Networks</a>
                 <span>The virtual network XML format</span>
-                <ul>
-                  <li>
-                    <a href="formatnwfilter.html">Network Filtering</a>
-                    <span>Network filter XML format</span>
-                  </li>
-                </ul>
+              </li>
+              <li>
+                <a href="formatnwfilter.html">Network Filtering</a>
+                <span>Network filter XML format</span>
               </li>
               <li>
                 <a href="formatstorage.html">Storage</a>