Errata: Introducing ReScript 12

Known errors in Introducing ReScript 12, second edition, by printed page number. Found something that isn't here? Let me know.

p. 80 — the guard in matchRecord4 should use if, not when:
```
| {name: n} if String.length(n) > 10 => true
```
p. 87 — manhattan2 should use Math.Int.abs rather than a bare abs. (Math.abs on p. 86 is correct; that example uses floats.)
p. 92 — “by having Array.get returns an optional value” should read “return an optional value.”
p. 104 — Array.partition is in the standard library as of ReScript 12.2. Disregard the note saying it is Belt-only.
p. 157 — ArrayStack.peek is missing its if. Since array access already returns an option, the check can go entirely:
```
let peek = stack => stack[Array.length(stack) - 1]
```
p. 175 — TAG holds the constructor name as a string, so the two compiled listings should read TAG: "Player" and TAG: "Enemy". The raw JavaScript and console output on those pages are already correct.

DANNY YANG