nixos/doc: Fix spurious indentation

nixos/doc/manual/configuration/modularity.xml

@@ -129,17 +129,17 @@ nix-repl> map (x: x.hostName) config.<xref linkend="opt-services.httpd.virtualHo
   While abstracting your configuration, you may find it useful to generate
   modules using code, instead of writing files. The example below would have
   the same effect as importing a file which sets those options.
-<screen>
-     { config, pkgs, ... }:
+<programlisting>
+{ config, pkgs, ... }:
 
-     let netConfig = { hostName }: {
-       networking.hostName = hostName;
-       networking.useDHCP = false;
-    };
+let netConfig = { hostName }: {
+  networking.hostName = hostName;
+  networking.useDHCP = false;
+};
 
-    in
+in
 
-    { imports = [ (netConfig "nixos.localdomain") ]; }
-  </screen>
+{ imports = [ (netConfig "nixos.localdomain") ]; }
+</programlisting>
  </para>
 </section>