Rollup merge of rust-lang#66360 - elichai:2019-11-vec-link, r=dtolnay

Fix link to Exten in Vec::set_len

Fixes rust-lang#66354

Unrelated to this PR, I think we should stop using `../../std/MODULE` and replace it with `../MODULE` that way if you're looking at docs in `core` or `alloc` clicking at a link won't forward you to `std`.

Author: JohnTitor

src/liballoc/vec.rs

@@ -861,7 +861,7 @@ impl<T> Vec<T> {
     ///
     /// [`truncate`]: #method.truncate
     /// [`resize`]: #method.resize
-    /// [`extend`]: #method.extend-1
+    /// [`extend`]: ../../std/iter/trait.Extend.html#tymethod.extend
     /// [`clear`]: #method.clear
     ///
     /// # Safety