DOC: better help for sct_straighten_spinalcord
DOC: better help for sct_straighten_spinalcord

Former-commit-id: 82007c9f7061bbbb017f3b2a5dd2e4b7b3641a9d
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox