[gtk/antoniof-master-patch-32022] docs: Mark heading in migration guide

[António Fernandes <antoniof src gnome org>]

commit a810f600bfa3ed8340ce8e1799257e7cfa7d2aeb
Author: António Fernandes <antoniojpfernandes gmail com>
Date:   Sun Feb 14 15:20:54 2021 +0000

    docs: Mark heading in migration guide
    
    This heading was missing from the index because it was not marked as one

 docs/reference/gtk/migrating-3to4.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index 597e77c78e..c1670e4e2f 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -135,7 +135,7 @@ These functions are not supported in GTK 4. Instead, either use
 backend-specific APIs, or render your widgets using
 #GtkWidgetClass.snapshot() (once you are using GTK 4).
 
-Stop using GtkButton's image-related API
+### Stop using GtkButton's image-related API
 
 The functions and properties related to automatically add a GtkImage
 to a GtkButton, and using a GtkSetting to control its visibility, are