doc: Add missing backend_type to pg_stat_activity

Commit 0c679464a8 added the missing backendDesc for B_STANDALONE_BACKEND
but missed updating the list of backend types in the documentation.  Fix
by adding it to the list.

Author: Noriyoshi Shinoda <noriyoshi.shinoda@hpe.com>
Discussion: https://postgr.es/m/DM4PR84MB1734ECEA02BCB59564E8FC03EE4A9@DM4PR84MB1734.NAMPRD84.PROD.OUTLOOK.COM

doc/src/sgml/monitoring.sgml

@@ -988,7 +988,7 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
        <literal>logical replication worker</literal>,
        <literal>parallel worker</literal>, <literal>background writer</literal>,
        <literal>client backend</literal>, <literal>checkpointer</literal>,
-       <literal>archiver</literal>,
+       <literal>archiver</literal>, <literal>standalone backend</literal>,
        <literal>startup</literal>, <literal>walreceiver</literal>,
        <literal>walsender</literal> and <literal>walwriter</literal>.
        In addition, background workers registered by extensions may have