OPT: detect_vertebral_levels: minor change
OPT: detect_vertebral_levels: minor change

Former-commit-id: 318a5a5e2a821d8876f68b0f03c58e7e418049f3
Former-commit-id: 6bd7e2d97424804008b69245b8a575bba27d5877
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox