doc: change pg_upgrade wal_level to be not minimal

Previously it was specified to be only replica.

Discussion: https://postgr.es/m/20200618180058.GK7349@momjian.us

Backpatch-through: 9.5

Author: bmomjian

doc/src/sgml/ref/pgupgrade.sgml

@@ -360,8 +360,8 @@ NET STOP postgresql-&majorversion;
      <quote>Latest checkpoint location</quote> values match in all clusters.
      (There will be a mismatch if old standby servers were shut down
      before the old primary or if the old standby servers are still running.)
-     Also, change <varname>wal_level</varname> to
-     <literal>replica</literal> in the <filename>postgresql.conf</filename> file on the
+     Also, make sure <varname>wal_level</varname> is not set to 
+     <literal>minimal</literal> in the <filename>postgresql.conf</filename> file on the
      new primary cluster.
     </para>
    </step>