fix problematic xrefs
<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>
