DOC: improving the documentation of sct_straighten_spinalcord (-speed_factor)
DOC: improving the documentation of sct_straighten_spinalcord (-speed_factor)

Former-commit-id: 4bd6ae8de0eafd25591797a8a195d90413afb03b
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox