[gtkmm-documentation] index-in.docbook: Update the link to the DTD file

[Kjell Ahlstedt <kjellahl src gnome org>]

commit d3b1897d3720ea6a0996d4dd19e5efcb193223f4
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed Mar 17 11:34:26 2021 +0100

    index-in.docbook: Update the link to the DTD file
    
    Fixes #14

 docs/tutorial/C/index-in.docbook | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/docs/tutorial/C/index-in.docbook b/docs/tutorial/C/index-in.docbook
index 559cdfc..36b0e8f 100644
--- a/docs/tutorial/C/index-in.docbook
+++ b/docs/tutorial/C/index-in.docbook
@@ -1,6 +1,6 @@
 <?xml version="1.0"?>
 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
-  "http://docbook.org/docbook/xml/4.5/docbookx.dtd"; [
+  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"; [
   <!ENTITY url_refdocs_base_glib_html "http://developer.gnome.org/glibmm/unstable/";>
   <!ENTITY url_refdocs_base_glib "&url_refdocs_base_glib_html;classGlib_1_1">
   <!ENTITY url_refdocs_base_gio "&url_refdocs_base_glib_html;classGio_1_1">
@@ -35,6 +35,7 @@ tools (itstool and friends) can't create translated index.docbook files if
 a <programlisting> element occurs in a <para> element.
 <programlisting> can be a direct child of e.g. <chapter>, <sect1>, <sect2>,
 <sect3>, <sect4>, <sect5>.
+See https://gitlab.gnome.org/GNOME/gtkmm-documentation/-/merge_requests/11
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