[llvm] [Github] Escape `@` and html in the <details> block (PR #66118)

[via llvm-commits]

cor3ntin wrote:

<pre lang="diff">
+ class {patch}
- bar
</pre>

> I wonder whether `#` needs to be escaped as well

Yes, done!

https://github.com/llvm/llvm-project/pull/66118