MAINT Fix typos and wording across the mooc

[ArturoAmorQ]

As per RL feedback from @tuscland, in this PR:

• corrects a typo "metioned" -> "mentioned";
• changes idiomatic expression "to linger on" -> "to focus on";
• adds a comment on overfitting when random forest are not correctly tuned;
• adds link towards wikipedia to explain the concept of "expected value" as a generalization of the weighted average.

[tuscland]

Thank you Arturo for taking in account my feedback, and congrats to the MOOC team for this excellent training!

[glemaitre]

LGTM