Fixing some typos in the vignette

Finlay, I have begun looking at mizer and I am impressed. The vignette is extremely well written and useful and I am working myself through the examples. While reading I came across some trivial typos. I thought fixing them would be a good pretext for getting in touch. It is not worth your time looking at them in detail, except perhaps the changes in equation (3.4) and (3.9).

One other thing: the function $\mu_p(w)$ in equation (3.15) appears not to be defined anywhere. One might guess that it is defined similar to the $\mu_p.i$ in (3.12), but what to use in place of the $f_i$ that appears in that equation?