Fix incorrect example of to_timestamp() usage.

Must use HH24 not HH to read a hour value exceeding 12.

This was already fixed in HEAD in commit d3cd36a, but I didn't think
of backpatching it.

Report: https://postgr.es/m/20161229170043.10139.21416@wrigleys.postgresql.org

Author: tglsfdc

doc/src/sgml/func.sgml

@@ -6230,7 +6230,7 @@ SELECT regexp_matches('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
       <para>
        Here is a more
        complex example:
-       <literal>to_timestamp('15:12:02.020.001230', 'HH:MI:SS.MS.US')</literal>
+       <literal>to_timestamp('15:12:02.020.001230', 'HH24:MI:SS.MS.US')</literal>
        is 15 hours, 12 minutes, and 2 seconds + 20 milliseconds +
        1230 microseconds = 2.021230 seconds.
       </para>