nixos/doc: Improve code listings

By adding prompts and replaceables and removing unnecessary indentation.

nixos/doc/manual/installation/upgrading.xml

@@ -67,32 +67,32 @@
   <literal>nixos-20.03</literal> channel. To see which NixOS channel you’re
   subscribed to, run the following as root:
 <screen>
-# nix-channel --list | grep nixos
+<prompt># </prompt>nix-channel --list | grep nixos
 nixos https://nixos.org/channels/nixos-unstable
 </screen>
   To switch to a different NixOS channel, do
 <screen>
-# nix-channel --add https://nixos.org/channels/<replaceable>channel-name</replaceable> nixos
+<prompt># </prompt>nix-channel --add https://nixos.org/channels/<replaceable>channel-name</replaceable> nixos
 </screen>
   (Be sure to include the <literal>nixos</literal> parameter at the end.) For
   instance, to use the NixOS 20.03 stable channel:
 <screen>
-# nix-channel --add https://nixos.org/channels/nixos-20.03 nixos
+<prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-20.03 nixos
 </screen>
   If you have a server, you may want to use the “small” channel instead:
 <screen>
-# nix-channel --add https://nixos.org/channels/nixos-20.03-small nixos
+<prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-20.03-small nixos
 </screen>
   And if you want to live on the bleeding edge:
 <screen>
-# nix-channel --add https://nixos.org/channels/nixos-unstable nixos
+<prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-unstable nixos
 </screen>
  </para>
  <para>
   You can then upgrade NixOS to the latest version in your chosen channel by
   running
 <screen>
-# nixos-rebuild switch --upgrade
+<prompt># </prompt>nixos-rebuild switch --upgrade
 </screen>
   which is equivalent to the more verbose <literal>nix-channel --update nixos;
   nixos-rebuild switch</literal>.