xdg-user-dirs: fix manpage rendering

Previously, `man xdg-user-dirs-update` contained this:

```
--set NAME PATH
   Sets the XDG user dir with the given name.

   NAME should be one of the following:

       PATH must be an absolute path, e.g.  $HOME/Some/Directory.
```

Now, it's correctly rendered:

```
--set NAME PATH
   Sets the XDG user dir with the given name.

   NAME should be one of the following:
       DESKTOP
       DOWNLOAD
       TEMPLATES
       PUBLICSHARE
       DOCUMENTS
       MUSIC
       PICTURES
       VIDEOS

   PATH must be an absolute path, e.g.  $HOME/Some/Directory.
```

pkgs/by-name/xd/xdg-user-dirs/package.nix

@@ -4,6 +4,7 @@
   fetchurl,
   libxslt,
   docbook_xsl,
+  docbook_xml_dtd_43,
   gettext,
   libiconv,
   makeWrapper,
@@ -24,6 +25,7 @@ stdenv.mkDerivation (finalAttrs: {
     makeWrapper
     libxslt
     docbook_xsl
+    docbook_xml_dtd_43
   ] ++ lib.optionals stdenv.hostPlatform.isDarwin [ gettext ];
 
   preFixup = ''