[Chapter 66] `SpellUseSystem` is never called

https://github.com/amethyst/rustrogueliketutorial/blob/33872fe582f226178436847e1f74eafcbf9c0d1a/book/src/chapter_66.md?plain=1#L409 The text of the chapter describes how to write `SpellUseSytem` but forgets to add it to the `run_systems` function in `main.rs`.