docs: add derive for struct

Code blocks in doc comments are compiled and run, so we show `Copy` works in this example.

Co-authored-by: Poliorcetics <poliorcetics@users.noreply.github.com>

library/core/src/marker.rs

@@ -323,6 +323,7 @@ pub trait StructuralEq {
 /// ```
 /// # #![allow(dead_code)]
 /// # struct PointList;
+/// #[derive(Copy, Clone)]
 /// struct PointListWrapper<'a> {
 ///     point_list_ref: &'a PointList,
 /// }