pgsql: docs: fix wording of REFRESH CONCURRENTLY manual page

docs: fix wording of REFRESH CONCURRENTLY manual page

Reported-by: Asim / apraveen@pivotal.io

Discussion: /messages/by-id/157076828181.26165.15231292023740913543@wrigleys.postgresql.org

Backpatch-through: 9.4

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4e3be56984a66d2bcc5aa0e6a2a0d1feb104502f

Modified Files
--------------
doc/src/sgml/ref/refresh_materialized_view.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)