MAINTAINERS: unify reference to xen-devel list

In the linux kernel MAINTAINERS file, largely
  "xen-devel@lists.xenproject.org (moderated for non-subscribers)"
is used to refer to the xen-devel mailing list.

The DRM DRIVERS FOR XEN section entry mentions
xen-devel@lists.xen.org instead, but that is just the same
mailing list as the mailing list above.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

MAINTAINERS

@@ -5092,7 +5092,7 @@ DRM DRIVERS FOR XEN
 M:	Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
 T:	git git://anongit.freedesktop.org/drm/drm-misc
 L:	dri-devel@lists.freedesktop.org
-L:	xen-devel@lists.xen.org
+L:	xen-devel@lists.xenproject.org (moderated for non-subscribers)
 S:	Supported
 F:	drivers/gpu/drm/xen/
 F:	Documentation/gpu/xen-front.rst