sct_register_to_template: Fix '%' in argparse help links (#2928)
sct_register_to_template: Fix '%' in argparse help links (#2928)

% is used for another purpose in argparse, and must be changed to %% to render correctly.
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox