[libcxx][docs] Clearer mark up for --param

Summary:
When I read this on the website it looks like the `--` in the used font turns
into an em dash. I updated this with inline literal mark up so the `--` will
remain obvious.

Reviewers: EricWF, #libc!

Subscribers: libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D78405