You can get the Stan code using
stancode <- rstan::get_stancode(mod$stanfit)
cat(stancode)
or by doing
and clicking on More and then Model Code.
However, the Stan code for stan_glm (and the other modeling functions) is long and complicated because it is flexible enough to be used for all the different model formulas, priors, other options, specified by the user. So it won't look like a simple Stan model for a glm, but I guess it could be helpful to you if you're interested in some of the tricks for numerical stability, speed, etc, that we've used. If you have any questions about it let us know!
Hope that helps,
Jonah