Fix RelativeURLs plugin.

[lovelydinosaur]

This isn't ideal, because...

• The project's running up against the obvious lack of a test suite.
• It's a quick-fix rather than something that's intentionally part of the public API.
• There's a weird dependency cycle developing both mkdocs and httpx-redesign (or whatever it's called)

Even so, I'm going to just get this into a releasable state. The projects both have a similar minimalism, and it's super important to have a live project to work against with the server.

Those issue ought to get into more of a relaxed state as the projects mature.
There's still just a little bit of chicken-and-egg going on here.

[lovelydinosaur]

This was working correctly within the mkdocs project serving it's own documentation, so the breakage wasn't immediately obvious.