Copy-editing the "Arrow Functions and their scope" article

Fixed some typos, ambiguities, improved formatting.

Please take a look, but don't merge yet until the comments marked as TODO are resolved. I need some feedback/discussion on the TODOs.

Also, as I'm using inline code blocks (backticks), this PR is blocked by #7.

I'll send the PR to master once all of this is settled.
cc @felipenmoura