fix and explain why the date range to estimate secondary observations

Change

```r
# Estimate from first 60 days of this data
cases_to_estimate <- reported_cases_deaths[31:60, ]
```

To

```r
# Estimate from day 31 to day 60 of this data
cases_to_estimate <- reported_cases_deaths[31:60, ]
```

Plus, explanation why: between day 1 and day 30, there are no deaths reported!

To fix in #47 