Fix type of $public_id of libxml_set_external_entity_loader (#2592)

Author: nielsdos

reference/libxml/functions/libxml-set-external-entity-loader.xml

@@ -30,7 +30,7 @@
       A <type>callable</type> with the following signature:
       <methodsynopsis>
        <type class="union"><type>resource</type><type>string</type><type>null</type></type><methodname><replaceable>resolver</replaceable></methodname>
-       <methodparam><type>string</type><parameter>public_id</parameter></methodparam>
+       <methodparam><type class="union"><type>string</type><type>null</type></type><parameter>public_id</parameter></methodparam>
        <methodparam><type>string</type><parameter>system_id</parameter></methodparam>
        <methodparam><type>array</type><parameter>context</parameter></methodparam>
       </methodsynopsis>