Add missing lifetime specifier

Author: jeremystucki

src/libcore/marker.rs

@@ -498,7 +498,7 @@ macro_rules! impls{
 /// #     end: *const T,
 /// #     phantom: PhantomData<&'a T>,
 /// # }
-/// fn borrow_vec<T>(vec: &Vec<T>) -> Slice<T> {
+/// fn borrow_vec<T>(vec: &Vec<T>) -> Slice<'_, T> {
 ///     let ptr = vec.as_ptr();
 ///     Slice {
 ///         start: ptr,