Missing code for `set_blocked`

https://github.com/amethyst/rustrogueliketutorial/blob/33872fe582f226178436847e1f74eafcbf9c0d1a/book/src/chapter_57a.md?plain=1#L133

There is not text describing the code for `set_blocked` which is used in the snippet above. It's straightforward to implement and gets removed later in the chapter, but I think it should be added for completeness/clarity (as it's additional confusion to have to search and check if you missed something while reading the chapter).