INST: installer: clarified if SCT already installed
INST: installer: clarified if SCT already installed

Former-commit-id: 7b15ef912f1c1e8d16a4846041cc48d0ed875e9a
Former-commit-id: 3830ed4e49c9284d308b2a5be5b4d7a006744f6c
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox