Incomplete Weekend Drinking dataset in Subsection 4.10

Hi, 
Seems the Weekend Drinking dataset ([`weekendDrinks.csv`](https://github.com/proback/BeyondMLR/blob/master/data/weekendDrinks.csv)) is incomplete used in [Subsection 4.10](https://bookdown.org/roback/bookdown-BeyondMLR/ch-poissonreg.html#cs:drinking).
The provided data does not contain `firstYear` column.
Thanks!