docs: add another derive for Copyable struct

This adds another `derive` for a `Copy`able struct, so that we are
consistent with `derive` annotations.

library/core/src/marker.rs

@@ -291,6 +291,7 @@ pub trait StructuralEq {
 ///
 /// ```
 /// # #[allow(dead_code)]
+/// #[derive(Copy, Clone)]
 /// struct Point {
 ///    x: i32,
 ///    y: i32,