rustbook: Generics chapter trait confusion

[Munksgaard]

The generics chapter ends like this:

To fix this example, we need to learn about another Rust feature: traits.

But the generics chapter (5.23) comes after the traits chapter (5.21). However, there is a trait object chapter (5.25) that comes after the generics chapter.

[steveklabnik]

#24571 Will fix this. Thanks!