Rollup merge of #70730 - yoshuawuyts:fix-wake-docs-link, r=Dylan-DPC
Rollup merge of #70730 - yoshuawuyts:fix-wake-docs-link, r=Dylan-DPC

Fix link in task::Wake docs

`task::Wake` was introduced in https://github.com/rust-lang/rust/pull/68700. While I was browsing the docs I noticed a link to `sync::Arc` wasn't resolving correctly. This patch fixes that. Thanks!

## Before

![Screenshot_2020-04-03 std task Wake - Rust](https://user-images.githubusercontent.com/2467194/78346384-466cb280-759f-11ea-97c8-aede186c674e.png)

## Proposed

![Screenshot_2020-04-03 alloc task Wake - Rust](https://user-images.githubusercontent.com/2467194/78349819-79657500-75a4-11ea-9282-16691ae2a5d4.png)

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust