Fix one missing dyn.

It's in the documentation of `Unsize`.

src/libcore/marker.rs

@@ -103,7 +103,7 @@ pub trait Sized {
 /// `Unsize` is implemented for:
 ///
 /// - `[T; N]` is `Unsize<[T]>`
-/// - `T` is `Unsize<Trait>` when `T: Trait`
+/// - `T` is `Unsize<dyn Trait>` when `T: Trait`
 /// - `Foo<..., T, ...>` is `Unsize<Foo<..., U, ...>>` if:
 ///   - `T: Unsize<U>`
 ///   - Foo is a struct