docs: Mark heading in migration guide

This heading was missing from the index because it was not marked as one

diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index 597e77c78ec5832f1a01fb8a7b1195ca5d5b74f2..c1670e4e2f7b22f086642a6c5750bcf3ad4a2be9 100644 (file)
--- 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