Book content contradicts state of the codebase

https://github.com/amethyst/rustrogueliketutorial/blob/33872fe582f226178436847e1f74eafcbf9c0d1a/book/src/chapter_63.md?plain=1#L247

At this point in the project, the `tile_content` field has been removed due to a previous refactor (extracting the `/spatial` folder).
This loop needs to be replaced with the `for_each_tile_content` helper.

UPDATE: there seems to be other occurrences of this in the following chapters too.