Chapter 15 small typo

[CorwinCheung]

Chapter name: NP, NP completeness, and the Cook-Levin Theorem

List of bugs/typos

1. First paragraph of chapter: "It turns out we can reduce all three problems to 3SAT in one fell swoop." should be "It turns out we can reduce all four problems to 3SAT in one fell swoop." given that there were 4 problems listed in the first sentence.