Doc: fix broken markup.

Commit 3d956d95 was apparently not checked against HEAD's doc toolchain.
Per buildfarm.

doc/src/sgml/ddl.sgml

@@ -3038,7 +3038,7 @@ VALUES ('Albany', NULL, NULL, 'NY');
 
    <para>
     Partitions can also be foreign tables, although they have some limitations
-    that normal tables do not; see <xref linkend="sql-createforeigntable"> for
+    that normal tables do not; see <xref linkend="sql-createforeigntable"/> for
     more information.
    </para>