This is just a mechanincal change, so that the public documentation[*]
refers to the latest release Fedora versions, instead of the EOLed
versions.
While at it, also update the `virt-cutomize` Makefile.am
[*] http://libguestfs.org/virt-builder.1.html
Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>