guide: Add `-p` flag to `mkdir` — radicle.xyz

To create parent directories if needed for people that are missing them.

 > If you are a **macOS** user, install Tor via `brew install tor` and restart
 > it via `brew services restart tor`.
 >
 > Use `mkdir -m 700 ~/.tor/radicle` to create your onion service directory.
 > Use `mkdir -p -m 700 ~/.tor/radicle` to create your onion service directory.
 >
 > The Tor configuration file is located at `/usr/local/etc/tor/torrc` and your
 > `.onion` address can be found in `~/.tor/radicle/hostname`.