fix problematic xrefs

gpdb-doc/dita/admin_guide/external/g-external-tables.xml

@@ -65,7 +65,7 @@
                <li id="du210102">Reading external table data in parallel 
                from multiple Greenplum database segment instances to optimize large load operations;</li>
                <li id="du210103">Filter pushdown (if a query contains WHERE clause, 
-               it may be passed to the external data source). See <xref href="../../../ref_guide/config_params/guc-list.xml#gp_external_enable_filter_pushdown">.
+               it may be passed to the external data source). See <xref href="../../ref_guide/config_params/guc-list.xml#gp_external_enable_filter_pushdown"></xref>.
                Note that this feature is currently supported only by the <codeph>pxf</codeph> protocol (see <xref href="g-pxf-protocol.xml"></xref>).</li>
             </ul>
             <p>Readable external tables allow only <codeph>SELECT</codeph> operations.</p>