Improve LoKi Functors lesson

From Vanya:

> In https://lhcb.github.io/first-analysis-steps/loki-functors.htmlt,  one probably can avoid a long paragraph with discussions on VFASPF,
> 
> there are functors  CHI2VX and CHI2VXNDOF  that get particle as an argument and  evaluate corresponding chi2_VX  ( or chi2_VX/ndf).
> 
> I think it could simplify a bit the material.
> 