fix apt.yml include

In status-site, this include wasn't working so we were getting that
error:

    !reference [".apt-init"] could not be found

In status-site@d4e39246, we remove the
slash which fixed this problem, so presumably this will fix it for all
includes.

I bet this wasn't detected because the lektor.yml file also includes
apt.yml, but without this leading slash.

One would have thought the include itself would have generated an
error...