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

Change-Id: I5f869c58c2601a9d572a053cf84af793e0c07238
Reviewed-on: http://review.couchbase.org/37093
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>