Rollup merge of #40689 - GuillaumeGomez:rustdoc-associated-type-formatting, r=frewsxcv

Add whitespace around "=" in assoc items

Part of #40641.

r? @rust-lang/docs

Before:

<img width="1440" alt="screen shot 2017-03-20 at 22 42 34" src="https://cloud.githubusercontent.com/assets/3050060/24123102/89181d8c-0dbe-11e7-897c-841497cf7001.png">

After:

<img width="1440" alt="screen shot 2017-03-20 at 22 42 36" src="https://cloud.githubusercontent.com/assets/3050060/24123118/8dec176e-0dbe-11e7-9759-cabbd062a4c2.png">