[RF] Don't use RooTruthModel in `rf704_aplitudefit` tutorial

The RooTruthModel can't be used in that test, because it will always
return zero for negative observable values.

The point of the tutorial is to demostrate that the fit can deal with
negative amplitude values for negative observable values, so this
feature of the RooTruthModel is not helpful here.

It was only a coincidence that the tutorial still worked: there was a
typo in the RooFormulaVars (extra whitespace), causing the RooTruthModel
to just relay to the original formula instead of using the compiled code
that returns zero for negative observable values.