docs: use consistent markup for PostgreSQL

"PostgreSQL" should use <productname> markup consistenktly, so that
if we do apply styling on it it will be consistently applied.  Fix
by renaming the one exception to the rule.

Discussion: https://postgr.es/m/F2EF5217-27A3-4962-9AE5-2E6C2CB3D0FF@yesql.se

doc/src/sgml/datatype.sgml

@@ -2672,7 +2672,7 @@ TIMESTAMP WITH TIME ZONE '2004-10-19 10:23:54+02'
      To complicate matters, some jurisdictions have used the same timezone
      abbreviation to mean different UTC offsets at different times; for
      example, in Moscow <literal>MSK</literal> has meant UTC+3 in some years and
-     UTC+4 in others.  <application>PostgreSQL</application> interprets such
+     UTC+4 in others.  <productname>PostgreSQL</productname> interprets such
      abbreviations according to whatever they meant (or had most recently
      meant) on the specified date; but, as with the <literal>EST</literal> example
      above, this is not necessarily the same as local civil time on that date.