sct_extract_metric: Fixed method max (#2087)
sct_extract_metric: Fixed method max (#2087)

Fixed #2086

Former-commit-id: 3c0334216099c383b4c8f7efa126e1fc0ef9ae33
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox