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

Hi everyone,

I think there is an issue in the [exercise notebook](https://github.com/GoogleCloudPlatform/training-data-analyst/blob/master/quests/vertex-ai/vertex-bqml/lab_exercise.ipynb) 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