[gtk+] Add a bit more to the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 53fc5a7d83039f85953d5bbe3fafe6a70ed80e5a
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Oct 20 14:05:34 2016 -0400

    Add a bit more to the migration guide

 docs/reference/gtk/migrating-2to4.xml |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-2to4.xml b/docs/reference/gtk/migrating-2to4.xml
index 6083338..12e6814 100644
--- a/docs/reference/gtk/migrating-2to4.xml
+++ b/docs/reference/gtk/migrating-2to4.xml
@@ -9,7 +9,7 @@
   <para>
     If your application is still using GTK+ 2, you should first convert it to
     GTK+ 3, by following the <ulink 
url="https://developer.gnome.org/gtk3/stable/gtk-migrating-2-to-3.html";>migration guide</ulink> in the GTK+ 3
-    documentation.
+    documentation, and then follow <xref linkend="gtk-migrating-3-to-4"/>.
   </para>
 
 </chapter>