[gtkmm-documentation/gtkmm-3-24] index-in.docbook: Update the link to the DTD file

[Kjell Ahlstedt <kjellahl src gnome org>]

commit d95fe917a94326c7587e60b3f38f9e2d5a318097
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed Mar 17 11:29:03 2021 +0100

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

 docs/tutorial/C/index-in.docbook | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/docs/tutorial/C/index-in.docbook b/docs/tutorial/C/index-in.docbook
index d666dbe..1d8eab3 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/2.64/";>
   <!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">