[totem] build: Fix build with gtk-doc 1.26

[Jeremy Bicha <jbicha src gnome org>]

commit 6bf1e1d06d1a93b727f2fa7a4ba5c6dae2977199
Author: Jeremy Bicha <jbicha ubuntu com>
Date:   Thu Oct 19 10:41:55 2017 -0400

    build: Fix build with gtk-doc 1.26
    
    https://bugzilla.gnome.org/show_bug.cgi?id=789202

 docs/reference/meson.build |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/docs/reference/meson.build b/docs/reference/meson.build
index 16f64bb..ff61274 100644
--- a/docs/reference/meson.build
+++ b/docs/reference/meson.build
@@ -89,7 +89,7 @@ gnome.gtkdoc(
     '--ignore-headers=' + ' '.join(private_headers),
   ],
   mkdb_args: [
-    '--sgml-mode',
+    '--xml-mode',
     '--output-format=xml',
     '--name-space=' + meson.project_name()
   ],