BUG: fixed bug in usage
BUG: fixed bug in usage

Former-commit-id: a29320b432621d805958e3bba8f5af92963f5a16
Former-commit-id: b0734ff8f588d37c632e6c16b7ea2dcb8c6847b5
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox