doc: fix another case of missing productname markup

As a follow-up commit to 0f85db9, this adds <productname> markup
to another case of "PostgreSQL".

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Discussion: https://postgr.es/m/TYAPR01MB58667A7C8317E267467CC599F5869@TYAPR01MB5866.jpnprd01.prod.outlook.com

Author: danielgustafsson

doc/src/sgml/func.sgml

@@ -7473,7 +7473,7 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
       <thead>
        <row>
         <entry>SQL standard</entry>
-        <entry>PostgreSQL</entry>
+        <entry><productname>PostgreSQL</productname></entry>
        </row>
       </thead>