A tiny error in "Atomics"

In [Atomics](https://github.com/rust-lang/nomicon/blob/master/src/atomics.md),

```markdown
However there's a third potential state that the hardware enables:

* `y = 2`: (thread 2 saw `x = 1`, but not `y = 3`, and then overwrote `y = 3`)
```

the last `y = 3` should be `y = 2`.