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>

man/systemd-nspawn.xml

@@ -1,8 +1,8 @@
 <?xml version='1.0'?>
 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
-<!ENTITY fedora_latest_version "28">
-<!ENTITY fedora_cloud_release "1.1">
+<!ENTITY fedora_latest_version "30">
+<!ENTITY fedora_cloud_release "1.2">
 ]>
 <!-- SPDX-License-Identifier: LGPL-2.1+ -->
 
@@ -1343,7 +1343,7 @@
 
       <programlisting># machinectl pull-raw --verify=no \
       https://download.fedoraproject.org/pub/fedora/linux/releases/&fedora_latest_version;/Cloud/x86_64/images/Fedora-Cloud-Base-&fedora_latest_version;-&fedora_cloud_release;.x86_64.raw.xz
-# systemd-nspawn -M Fedora-Cloud-Base-&fedora_latest_version;-&fedora_cloud_release;.x86_64.raw</programlisting>
+# systemd-nspawn -M Fedora-Cloud-Base-&fedora_latest_version;-&fedora_cloud_release;.x86_64</programlisting>
 
       <para>This downloads an image using
       <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>