add missing derive to fix compile error

src/libcore/cmp.rs

@@ -65,6 +65,7 @@ use self::Ordering::*;
 /// the same book if their ISBN matches, even if the formats differ:
 ///
 /// ```
+/// #[derive(PartialEq)]
 /// enum BookFormat {
 ///     Paperback,
 ///     Hardback,