doc: Fix title markup for AT TIME ZONE and AT LOCAL

The title for AT TIME ZONE and AT LOCAL was accidentally wrapping the
"and" in the <literal> tag.  Backpatch to v17 where it was introduced
in 97957fdbaa.

Author: Noboru Saito <noborusai@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Tatsuo Ishii <ishii@postgresql.org>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAAM3qn+7QUWW9R6_YwPKXmky0xGE4n63U3EsxZeWE_QtogeU8g@mail.gmail.com
Backpatch-through: 17

doc/src/sgml/func.sgml

@@ -10687,7 +10687,7 @@ SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-0
   </sect2>
 
   <sect2 id="functions-datetime-zoneconvert">
-   <title><literal>AT TIME ZONE and AT LOCAL</literal></title>
+   <title><literal>AT TIME ZONE</literal> and <literal>AT LOCAL</literal></title>
 
    <indexterm>
     <primary>time zone</primary>