REF: segment_graymatter: fixed display in usage
REF: segment_graymatter: fixed display in usage

Former-commit-id: d2dc5d5917eb6fb5ba786f7432ff39bb25b60cbd [formerly 577735dddc7a550e757ab91b5f52f117bba275f2]
Former-commit-id: 12e874eab94d4e04ef3b5ba15f99e4f8e7e3c1e6
Former-commit-id: 9f776b6d87e38bd4587d866fd054437a06ee2b01
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox