[Chapter 66] Wrong JSON data makes the game crash

https://github.com/amethyst/rustrogueliketutorial/blob/33872fe582f226178436847e1f74eafcbf9c0d1a/book/src/chapter_66.md?plain=1#L123-L134

At that point, `mana_cost` is declared as mandatory on the `Spell` struct in `spell_structs.rs`.
Since there is no `mana_cost` field in the JSON file, the game crashes on start when loading the JSON data.
The final state of the code for this chapter does include this `mana_cost` field.