[RF] Fix copy-paste error in rf205 tutorial and stressRooFit test

In the `rf205` tutorial and the associated stressRooFit test, the `bkg`
RooAddPdf was created using the `sig1frac`, while it is clearly the idea
to use the `bkg1frac` parameter.

This is fixed in this commit, also fixing the name of the `bkg1frac`
parameter, which was accidentally set to `sig1frac` (probably a
copy-paste error).

However, to not change the output of the tutorial and the stressRooFit
test, the value of `bkg1frac` was set to the same value as `sig1frac`,
such that this commit only fixes the model but doesn't change the
tutorial and test output.