fix pod error: too few backs!

> 1 POD Error
> The following errors were encountered while parsing the POD:
> Around line 98:
> You forgot a '=back' before '=head1'

Filesys::POSIX must have stolen this =back from Mail::Dir. Time to return to its
rightful owner.
fix repo resource in distro metadata
improve doc markup

pod doesn't care about the order of nested C<> and L<> but markdown does care
about the order of nested `` and [](): if `` is on the outside, it will treat
the []() as literal text and not a link