[gtk] gtk4-demo: Fix sliding puzzle demo

[Timm Bäder <baedert src gnome org>]

commit ae54517b33e6ea94b7e2228326e66860c0b02810
Author: Timm Bäder <mail baedert org>
Date:   Wed May 20 10:24:22 2020 +0200

    gtk4-demo: Fix sliding puzzle demo

 demos/gtk-demo/sliding_puzzle.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/demos/gtk-demo/sliding_puzzle.c b/demos/gtk-demo/sliding_puzzle.c
index dbe8129e60..098048eb5d 100644
--- a/demos/gtk-demo/sliding_puzzle.c
+++ b/demos/gtk-demo/sliding_puzzle.c
@@ -387,7 +387,7 @@ add_choice (GtkWidget    *container,
   icon = gtk_image_new_from_paintable (paintable);
   gtk_image_set_icon_size (GTK_IMAGE (icon), GTK_ICON_SIZE_LARGE);
 
-  gtk_box_append (GTK_BOX (container), icon);
+  gtk_flow_box_insert (GTK_FLOW_BOX (container), icon, -1);
 }
 
 GtkWidget *