Wrong icon for clipboard in reading chapter

### Problem

On https://rust-lang.github.io/mdBook/guide/reading.html#code-blocks, the icon for the clipboard is not correct. As of https://github.com/rust-lang/mdBook/pull/2421, it is now using an embedded icon, but the page still uses the old font-awesome icon.

@GuillaumeGomez Is there an easy way to reuse the icon there?


### Version

```text
mdbook v0.4.52
```