DOC: changed usage on sct_register_to_template.py
DOC: changed usage on sct_register_to_template.py

Former-commit-id: 4d8015c9a2d97820c44b466b345a0bd05f700114
Former-commit-id: 9f179d01f738ca62d65f9892db348931175b5d2b
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox