doc: Fix description of target_session_attrs=prefer-standby

If no standbys can be found in the set of connection points provided,
the fallback behavior is "any", and not "all" that does not exist.

Author: Greg Nancarrow
Reviewed-by: Laurenz Albe
Discussion: https://postgr.es/m/CAJcOf-fDaCv8qCpWH7k5Yh6zFxZeUwZowu4sCWQ2zFx4CdkHpA@mail.gmail.com

doc/src/sgml/libpq.sgml

@@ -1965,7 +1965,7 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
           <listitem>
            <para>
             first try to find a standby server, but if none of the listed
-            hosts is a standby server, try again in <literal>all</literal>
+            hosts is a standby server, try again in <literal>any</literal>
             mode
            </para>
           </listitem>