odocs: Clicking on `Mutex.t` link gives 404

[polytypic]

If you click the Mutex.t link in the Condition.await signature here, you get a 404 response.

[talex5]

@jonludlam I see this locally too (with dune build @doc). The link goes to Eio__/Mutex/index.html (which doesn't exist) but the documentation is in Eio/Mutex/index.html. The Eio_mutex.mli file has @canonical Eio.Mutex.