Section 1.9 missing some rename mentions

Some renames which were done in the source code were not mentioned in the book which causes confusion as the code snippets don't match up with the code from the previous chapter.

Specifically, the following renames are not mentioned in the book.
- structure `WantsToDrinkPotion` to `WantsToUseItem`
- field `potion` to `item` of the above structure
- variable `wants_drink` to `useitem` in the `run` method of `PotionUseSystem`

Also, the renames of all instances where the above were used in `inventory_system.rs` and `main.rs`.

Thank you for this amazing book !