CBD-1386:  fix typo in string concat

Change-Id: I867dc6ac1e5002404c465d64228c6f6f3d9c13e6
Reviewed-on: http://review.couchbase.org/39884
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>