OPT: detect_vertebral_levels: minor fix
OPT: detect_vertebral_levels: minor fix

Former-commit-id: e0cf713a4cc4d7e2bb7506925a0e12eb9552699c [formerly 7ed731357b9986f0224e531f7dd14c45f9f9bb5b]
Former-commit-id: 683518b99a89022706684e57b61627a91981d673
Former-commit-id: e26ad66fde47f80c1048526d6a96960f7e11c880
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox