CBLT-42:  fix typo in DOS:  "call :usage"

Change-Id: I3aa3a5ddca3c0398eec8c2bc278efa5de2555e97
Reviewed-on: http://review.couchbase.org/37094
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>