docs: bad command rendering on https://doc.rust-lang.org/cargo/commands/cargo-test.html

[dmitris]

Problem
https://doc.rust-lang.org/cargo/commands/cargo-test.html contains command where the first double-dash in cargo test -- --help is not rendered correctly - shows as a single dash. It is also paste-unfriendly - by pasting in the terminal the cursor goes several places back. The best would be to have real ASCII -- on the page.

Possible Solution(s)
Figure out how to make mbook to generate correct characters from the -- input.