Update the DTF lesson for D0 -> K- K+

[vlisovsk]

In the DTF lesson, there are some remnants from the time when we used an example the D0->K+pi- decay.

For example,

As an example let's say we want to examine the Cabibbo-suppressed decay of the D^0 into pi- pi+ instead of K- pi+.

A certain clean-up is needed to update these instructions.