Doc: Replace reference to pg_stat_wal_receiver.received_lsn by flushed_lsn

Oversight in 2c8dd05d, where the view's column has been renamed.

Reported-by: Fujii Masao
Discussion: https://postgr.es/m/c049ffcf-d2fe-90f7-c8ba-0741035aa6a7@oss.nttdata.com

doc/src/sgml/high-availability.sgml

@@ -906,7 +906,7 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
      On a hot standby, the status of the WAL receiver process can be retrieved
      via the <xref linkend="pg-stat-wal-receiver-view"/> view.  A large
      difference between <function>pg_last_wal_replay_lsn</function> and the
-     view's <literal>received_lsn</literal> indicates that WAL is being
+     view's <literal>flushed_lsn</literal> indicates that WAL is being
      received faster than it can be replayed.
     </para>
    </sect3>