Anonymous trait method parameters - src/generics/assoc_items/types.md

In accordance to RFC #1685, this code is invalid in 2018 edition:
```rust
trait Contains {
    type A;
    type B;

    fn contains(&self, &Self::A, &Self::B) -> bool;
}
```
and should be changed to:
```rust
trait Contains {
    type A;
    type B;

    fn contains(&self, _: &Self::A, _: &Self::B) -> bool;
}
```