kNN Regression: Maybe more motivation?

[nickeubank]

Reading over your kNN regression, I think we'll probably need more motivation on what you're trying to accomplish by "doing a regression" for those without more experience with that stuff.

[kylebradbury]

Done - let me know what you think of the revised version:

https://github.com/nickeubank/practicaldatascience_book/blob/main/notebooks/class_5/week_2/60_knn_for_regression.ipynb