Rollup merge of #82979 - GuillaumeGomez:run-button-pos, r=Nemo157

Rollup merge of #82979 - GuillaumeGomez:run-button-pos, r=Nemo157

Fix "run" button position in error index

This isn't really a rustdoc issue but I still made the same fix in the `rustdoc.css` file (doesn't hurt).

Before:

![Screenshot from 2021-03-10 16-35-49](https://user-images.githubusercontent.com/3050060/110655807-aa402800-81bf-11eb-8a88-bc979efd1697.png)

After:

![Screenshot from 2021-03-10 16-40-08](https://user-images.githubusercontent.com/3050060/110655843-b4622680-81bf-11eb-8670-42975d92b4eb.png)

cc ````@jyn514```` (considering this is quite a big bug and an easy fix)
r? ````@Nemo157````