Chapter 35: Entities not properly filtered out

Hi,

It looks like there is a bug in the retain closure when filtering out entities that would overlap with a room vault:

https://github.com/amethyst/rustrogueliketutorial/blob/33872fe582f226178436847e1f74eafcbf9c0d1a/chapter-35-vaults2/src/map_builders/prefab_builder/mod.rs#L385

I believe this should be:

```rust
let y = idx / width;
```