Typo on the Werk book

Hi there, I found your project on lobste.rs and was reading through the post and sifting through the book and found [this](https://simonask.github.io/werk/language/expressions.html?highlight=flatten#flatten):
```rs
let flattened = ["a", ["b", ["c"]]]   # ["a", "b", "c"]
```
This is probably supposed to be:
```rs
let flattened = ["a", ["b", ["c"]]] | flatten   # ["a", "b", "c"]
```