stan-reference: fix typo in naive bayes
stan-reference: fix typo in naive bayes

The parameters of the categorical distributions were named $\theta$ and $\phi$ everywhere else in the naive bayes section, not $\theta$ and $\pi$.
stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math