Small tweaks to vignette

Goal

A number of minor tweaks reflecting some feedback from @ChiragKumar9. Some of these might already be addressed so will mark as such

[ ] add a legend to hospital admissions plots indicating that the filled in points are the ones being used for calibration
[ ] fix documentation of indicate_ww_exclusions() (data argument is missing an ending to the sentence)
[ ] Small typo in Wastewater data pre-processing
[ ] Add enter so Parameters renders properly
[ ] Forecast date is mentioned twice in model specifications
[ ] "The" has a typo near line 239
[ ] Clarify what specification means, specifically that we expect discrete daily PMFs
[ ] Print additional sig figs in divergences (% of divergent transitions says 0 if < 20)