5tt2gmwmi: Fix -mask option
5tt2gmwmi: Fix -mask option

Error was made in porting to new syntax as part of 0.3.14, within commit ec49edcaf.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3