mrtrix3.image.statistic(): Fix parameter naming
mrtrix3.image.statistic(): Fix parameter naming

app.error() call contained a reference to the function being invoked, rather than the parameter that was passed to the function.

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