Wrong typing in chapter 57a

https://github.com/amethyst/rustrogueliketutorial/blob/33872fe582f226178436847e1f74eafcbf9c0d1a/book/src/chapter_57a.md?plain=1#L233

This should be `FnMut` instead of `Fn` and `mut f` instead of `f` , which matches the final code [here](https://github.com/amethyst/rustrogueliketutorial/blob/33872fe582f226178436847e1f74eafcbf9c0d1a/chapter-57a-spatial/src/spatial/mod.rs#L57C1-L58C24) (without that, `cargo run` will fail).