Update ModuleDescriptionParser.cxx

There are 3 possible choices for the type of transform (see link below), but the error message only shows 2: "linear" and "nonlinear"
https://github.com/Slicer/SlicerExecutionModel/blob/master/ModuleDescriptionParser/ModuleDescriptionParser.cxx#L894-L896
Merge pull request #25 from fbudin69500/patch-1

BUG: Missing "bspline" as a transform type choice in error message