Fixes to the `its_covid` notebook

[drbenvincent]

A few errors have crept in here. We've got a demo of custom priors in here, attempting to model the data in the original scale. But the model formula has not been updated, so it's still standardizing the data. We at the least we need to update the model formula to remove that. We can also remove the callout box about the results being standardized. And we can probably add some text in there now that the effects should be in the original scale (deaths).