Small edits to the start of the tutorial

Andrea. I'm loving this tutorial! Amazing work.

As I'm going through it I'm making small little edits for clarity and to make the English more idiomatic, and adding a few links.

I'll continue this as I go through the rest of the tutorial if you are interested in incorporating these edits. If you aren't interested, let me know so as not to waste anymore of my or your time.

Cheers!