BUG: sct_extract_metric: fixed bug related to saving text file
BUG: sct_extract_metric: fixed bug related to saving text file

Former-commit-id: 84d9e5986a025ff0f3d949b90a417bcc560ca0d4
Former-commit-id: 44f6d3f78eeec3769b28286f7d0d7e4bb0e6f8bc
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox