nixos/grub: Add another example for extraEntries

Someone on IRC wanted to boot Fedora from another disk. While I'm not
too familiar with UEFI booting in conjunction with GRUB2 it took some
time to get it to work.

So in order to safe others from frustration I'm adding this as another
example to the extraEntries option.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>