Liballoc extend use intra doc link

library/alloc/src/vec.rs

@@ -1610,8 +1610,6 @@ pub fn resize(&mut self, new_len: usize, value: T) {
     /// vec.extend_from_slice(&[2, 3, 4]);
     /// assert_eq!(vec, [1, 2, 3, 4]);
     /// ```
-    ///
-    /// [`extend`]: #method.extend
     #[stable(feature = "vec_extend_from_slice", since = "1.6.0")]
     pub fn extend_from_slice(&mut self, other: &[T]) {
         self.spec_extend(other.iter())