man: systemd-nspawn: Update syntax to launch an image

man: systemd-nspawn: Update syntax to launch an image

To access a shell on a disk image, the man page on Fedora-29 says to
run: `systemd-nspawn -M Fedora-Cloud-Base-28-1.1.x86_64.raw`.  Let's
try.

List existing images:

    $> machinectl list-images | awk '{print $1,$2}';
    NAME TYPE
    Fedora-Cloud-Base-30… raw

    1 images

Now invoke `systemd-nspawn` as noted in the man page:

    $> systemd-nspawn -M Fedora-Cloud-Base-30-1.2.x86_64.raw
    No image for machine 'Fedora-Cloud-Base-30-1.2.x86_64.raw'.

Removing the ".raw" extension launches the image and gives a shell.
Update the man page to reflect that.

Frantisek Sumsal on #systemd (Freenode) noted the reason: "In older
versions systemd -M accepted both image-name.raw and image-name as a
valid image names, however, on Fedora 29 (systemd-239) with all the
BTRFS stuff around it accepts only -M image-name (without the
extension)"

    - - -

While at it, update the fedora_{latest_version, cloud_release}
variables.

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>