fix rpg test case with insufficient mana
@@ -39,7 +39,7 @@ The `cast_spell` method takes a mutable reference to the Player as well as a `ma
```rust
let not_a_wizard_yet = Player { health: 79, mana: None, level: 9 };
assert_eq!(not_a_wizard_yet.cast_spell(5), 0)
- assert_eq!(not_a_wizard_yet.health, 70);
+ assert_eq!(not_a_wizard_yet.health, 74);
assert_eq!(not_a_wizard_yet.mana, None);
```
