Rollup merge of #78394 - rubik:master, r=m-ou-se

fix(docs): typo in BufWriter documentation

This PR fixes a small typo in the BufWriter documentation. The current documentation looks like this:

![2020-10-26-111501_438x83_scrot](https://user-images.githubusercontent.com/238549/97160357-83d3a000-177c-11eb-8a35-3cdd3a7d89de.png)

The `<u8>` at the end is mangled by Markdown. This PR makes the `BufWriter` documentation like the `BufReader` one:

https://github.com/rust-lang/rust/blob/master/library/std/src/io/buffered/bufreader.rs#L16

I'm tagging Steve as per the Rustc dev guide.

r? @steveklabnik