[Chapter 70] Missing text for `fire_on_target`

https://github.com/amethyst/rustrogueliketutorial/blob/33872fe582f226178436847e1f74eafcbf9c0d1a/chapter-70-missiles/src/player.rs#L54-L80
https://github.com/amethyst/rustrogueliketutorial/blob/master/chapter-70-missiles/src/player.rs#L482

The chapter 70 forgets to add the `fire_on_target` function and to call it from `player.rs` (to fire with the `F` key). The code for it is in the repo, but not mentioned in the book.