Fix return type of DOMNodeList::index()

DOMElement was missing until php/php-src#11342 fixed the return type.

Author: kocsismate

reference/dom/domnodelist/item.xml

@@ -10,7 +10,7 @@
  <refsect1 role="description">
   &reftitle.description;
   <methodsynopsis role="DOMNodeList">
-   <modifier>public</modifier> <type class="union"><type>DOMNode</type><type>DOMNameSpaceNode</type><type>null</type></type><methodname>DOMNodeList::item</methodname>
+   <modifier>public</modifier> <type class="union"><type>DOMElement</type><type>DOMNode</type><type>DOMNameSpaceNode</type><type>null</type></type><methodname>DOMNodeList::item</methodname>
    <methodparam><type>int</type><parameter>index</parameter></methodparam>
   </methodsynopsis>
   <para>