dwifslpreproc: Fix syntax bug following merge
dwifslpreproc: Fix syntax bug following merge

Erroneous merge in #1937 failed to account for parallel changes in Python API (#1449).

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