replace inv_gamma(0.001, 0.001) in getting started Rats example from BUGS

[bob-carpenter]

• fix the doc and output
• validate this issue is closed on example-models to fix the actual model: replace the inv_gamma(0.001, 0.001) priors in Rats example example-models#25

[davidmanheim]

Can we close this? (I don't think this examples is still included anywhere in Stan.)

[bob-carpenter]

It's a problem in all of our example models, but this isn't the repo where it'll get fixed.