`DESC` instead of `ASC` in vertex-bqml/lab_exercise.ipynb

[DiTo97]

Hi everyone,

I think there is an issue in the exercise notebook provided for the churn prediction quest in the Extract user demographic features section, as the description proposes to keep the demographic features associated to the first user engagement with the mobile app, but the SQL query orders the timestamps in descending order with DESC, thus taking the last user engagement