BUG: missed a - in the beta equation when no level
BUG: missed a - in the beta equation when no level

Former-commit-id: 2305c918f2c1e85df20733dcafcd83cd1cc21650
Former-commit-id: d75e2dbe61cc8f129586c9b3db2704aeba8fade8
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox