fix: typos in regression chapter

[kjappelbaum]

• Formally, standardization should only done on the training data (discussed nicely in Elements of Statistical Learning). For this reason, I would maybe do the split before the standardization.
• I'd perhaps be a bit less general with "L2 gives a better model and L1 gives a more interpretable result by zeroing features" as L1 also needs some tuning of lambda to make things really zero. Do you have some reference that shows that L2 give better performance than L1? My intuition would have been "it depends"
• an awesome resource for a deeper dive into model selection is https://arxiv.org/abs/1811.12808