Decorate WITH with literal markup tags

One instance of "WITH clause" was not using <literal> tags around
WITH, while others were, so add markup to the last one to ensure
consistency.  Backpatch to v15 where MERGE was added.

Reported-by: jian he <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CACJufxGJKY9ZCPV2WDM6xFsXq9C8r7r3vU6AkScN+p9k6CEpMw@mail.gmail.com
Backpatch-through: v15

doc/src/sgml/ref/merge.sgml

@@ -627,8 +627,8 @@ WHEN MATCHED THEN
     This command conforms to the <acronym>SQL</acronym> standard.
   </para>
    <para>
-    The WITH clause and <literal>DO NOTHING</literal> action are extensions to
-    the <acronym>SQL</acronym> standard.
+    The <literal>WITH</literal> clause and <literal>DO NOTHING</literal>
+    action are extensions to the <acronym>SQL</acronym> standard.
   </para>
  </refsect1>
 </refentry>