docs: unbreak spec build

docs: unbreak spec build

I've created a homebrew tap with sdop and xfpt in it, so I can install
those more easily on macOS in the future, and now have bothered actually
building the docs.  `.url()` should have been `&url()` in two places.

The `make spec.pdf` pipeline yields a document where those are not
clickable links, but if i use `make spec.ps` and let macOS auto-convert
to PDF upon open, those are proper clickable hyperlinks.  So this switch
is definitely for the better.