The documentation for `mdbook serve` references a `--websocket-hostname` command line option that no longer exists. It looks like the option was added in https://github.com/rust-lang/mdBook/pull/865 and removed in https://github.com/rust-lang/mdBook/pull/1211. Siimilar options are referenced in https://github.com/rust-lang/mdBook/pull/160, https://github.com/rust-lang/mdBook/pull/128, and https://github.com/rust-lang/mdBook/pull/298
