mrconvert: Update help information for -scaling option
mrconvert: Update help information for -scaling option

To reflect changes in 928cdc02.

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