clarify vignette

[n8thangreen]

With respect to the vignette, I am not really sure what the “formula used in the model” for MAIC and the corresponding R formula object passed as an argument to the strategy function represents.

One final point for the vignette is that some of the audience might view our “G-computation” approaches as a version of STC, so only labelling the “wrong” version of STC as STC might be confusing for them.