Cleanup post luafilesystem version de-underscoring (#2683)

The upstream luafilesystem tarball/version had a wayward/inconsistent
underscore in their more recent version tag.  The played badly with
our package fetching machinery (due to recent changes?).

Upstream cleaned up their bit which required some touchups here.

- updated the url
- updated the version (digest)
- updated the format statement for the path to the rockspec.