OPT: detect_vertebral_levels: fixed minor bug
OPT: detect_vertebral_levels: fixed minor bug

Former-commit-id: 26d7a82d7d76b6aad1c9389e6b7ce60508d8a953
Former-commit-id: 0cea3b8bd19c0117409bd9c4aac4ac8340f76afc
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox