Rollup merge of #85174 - GuillaumeGomez:doc-code-block-border-radius, r=jsha

Rollup merge of #85174 - GuillaumeGomez:doc-code-block-border-radius, r=jsha

Fix border radius for doc code blocks in rustdoc

In #85148, I made an invalid change on the border radius of the doc code blocks (look in the top left and bottom left corners of the code blocks).

Before this fix:

![Screenshot from 2021-05-11 11-14-59](https://user-images.githubusercontent.com/3050060/117791459-a4f86b80-b24a-11eb-8ac3-facc719c799a.png)

After this fix:

![Screenshot from 2021-05-11 11-05-29](https://user-images.githubusercontent.com/3050060/117791482-a9bd1f80-b24a-11eb-8c38-a01989595f5c.png)

r? `@jsha`