man: also add --enablerepo=updates to dnf invocation

Without the updates repo, we are installing packages from the time
that that version of Fedora was released. Normally, during the
lifetime of the release most packages are updated, so most of the
packages installed would be outdated, and the first update after
installation would update a massive set of packages. Avoid all this
by installing from the updates repo from the start.

Author: keszybz

man/systemd-nspawn.xml

@@ -850,7 +850,7 @@
     <example>
       <title>Build and boot a minimal Fedora distribution in a container</title>
 
-      <programlisting># dnf -y --releasever=23 --installroot=/srv/mycontainer --disablerepo=* --enablerepo=fedora install systemd passwd dnf fedora-release vim-minimal
+      <programlisting># dnf -y --releasever=23 --installroot=/srv/mycontainer --disablerepo=* --enablerepo=fedora --enablerepo=updates install systemd passwd dnf fedora-release vim-minimal
 # systemd-nspawn -bD /srv/mycontainer</programlisting>
 
       <para>This installs a minimal Fedora distribution into the