BUG: compute_snr: fixed variable name
BUG: compute_snr: fixed variable name

Former-commit-id: 2b37239817f2d47d6369b9d414fd28b4a3f0f100
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox