doc: Use prompt more often

nixos/modules/services/editors/emacs.xml

@@ -238,8 +238,8 @@ in
    <para>
     You can check that it works by executing this in a terminal:
 <screen>
-$ nix-build emacs.nix
-$ ./result/bin/emacs -q
+<prompt>$ </prompt>nix-build emacs.nix
+<prompt>$ </prompt>./result/bin/emacs -q
 </screen>
     and then typing <literal>M-x package-initialize</literal>. Check that you
     can use all the packages you want in this Emacs instance. For example, try
@@ -403,9 +403,9 @@ in [...]
    <para>
     To start the daemon, execute the following:
 <screen>
-$ nixos-rebuild switch  # to activate the new configuration.nix
-$ systemctl --user daemon-reload        # to force systemd reload
-$ systemctl --user start emacs.service  # to start the Emacs daemon
+<prompt>$ </prompt>nixos-rebuild switch  # to activate the new configuration.nix
+<prompt>$ </prompt>systemctl --user daemon-reload        # to force systemd reload
+<prompt>$ </prompt>systemctl --user start emacs.service  # to start the Emacs daemon
 </screen>
     The server should now be ready to serve Emacs clients.
    </para>