doc: clarify description for log_startup_progress_interval

Reported-by: Elena Indrupskaya

Discussion: https://postgr.es/m/0af43b49-1646-93d0-ccf1-bb3c635c8c6f@postgrespro.ru

Author: Elena Indrupskaya

Backpatch-through: 15

Author: bmomjian

doc/src/sgml/config.sgml

@@ -6752,7 +6752,7 @@ local0.*    /var/log/postgresql
          setting has the default value of 10 seconds, then a messages will be
          logged for syncing the data directory after it has been in progress
          for 10 seconds and again after it has been in progress for 20 seconds,
-         but nothing will be logged for resetting unlogged operations.
+         but nothing will be logged for resetting unlogged relations.
         </para>
        </listitem>
      </varlistentry>