Slice::into_vec: Don't link to Vec::into_boxed_slice

The documentation for this method appears on multiple different pages,
which causes the relative links to not always work.

src/libcollections/slice.rs

@@ -1411,9 +1411,7 @@ impl<T> [T] {
     /// Converts `self` into a vector without clones or allocation.
     ///
     /// The resulting vector can be converted back into a box via
-    /// the [`into_boxed_slice`] method.
-    ///
-    /// [`into_boxed_slice`]: vec/struct.Vec.html#method.into_boxed_slice
+    /// `Vec<T>`'s `into_boxed_slice` method.
     ///
     /// # Examples
     ///