doc: remove double-negative in REFRESH MATERIALIZED ... CONCURR

Reported-by: ap@robillo.net

Discussion: https://postgr.es/m/20170208152743.1411.6073@wrigleys.postgresql.org

Backpatch-through: master

Author: bmomjian

doc/src/sgml/ref/refresh_materialized_view.sgml

@@ -67,7 +67,7 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
       expression index or include a <literal>WHERE</literal> clause.
      </para>
      <para>
-      This option may not be used when the materialized view is not already
+      This option can only be used when the materialized view is already
       populated.
      </para>
      <para>