Fix invalid value of synchronous_commit in description of flush_lag

"remote_flush" has never been a valid user-facing value, but "on" is.

Author: Maksim Milyutin
Discussion: https://postgr.es/m/27b3b80c-3615-2d76-02c5-44566b53136c@gmail.com

doc/src/sgml/monitoring.sgml

@@ -1868,7 +1868,7 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       notification that this standby server has written and flushed it
       (but not yet applied it).  This can be used to gauge the delay that
       <literal>synchronous_commit</literal> level
-      <literal>remote_flush</literal> incurred while committing if this
+      <literal>on</literal> incurred while committing if this
       server was configured as a synchronous standby.</entry>
     </row>
     <row>