Minor fixes for talking-about-sets.lhs

Two additional comments:

- Just before "Proving Theorems With LiquidHaskell" you have a paragraph that begins with "Note: Oh quite." It's unclear what you're responding to there.
- I noticed that `[]` is given the type `{v: [{v:a | false}]}`, is that intended? I can see how that might be useful, the empty list has no elements so we can prove anything we want about them; just want to make sure the `false` isn't escaping into the environment and causing problems elsewhere.