[doc-gen] Vec::iter_mut is missing from the side list of the doc page

The page: https://doc.rust-lang.org/beta/std/vec/struct.Vec.html#method.iter

See the screenshot - missing `iter_mut` below `iter` in the side-list.

![image](https://user-images.githubusercontent.com/1288299/56848906-26a92500-6920-11e9-90e7-309c3ed93088.png)

