[gtk] gtk4-demo: Fix css_basics demo

[Timm Bäder <baedert src gnome org>]

commit 4b5a8c45b498114befb80917a5ff07f80dd0d0ea
Author: Timm Bäder <mail baedert org>
Date:   Wed May 20 10:27:00 2020 +0200

    gtk4-demo: Fix css_basics demo

 demos/gtk-demo/css_basics.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/demos/gtk-demo/css_basics.c b/demos/gtk-demo/css_basics.c
index cd51fc9d76..427ae45f1f 100644
--- a/demos/gtk-demo/css_basics.c
+++ b/demos/gtk-demo/css_basics.c
@@ -97,7 +97,7 @@ do_css_basics (GtkWidget *do_widget)
       container = gtk_scrolled_window_new (NULL, NULL);
       gtk_window_set_child (GTK_WINDOW (window), container);
       child = gtk_text_view_new_with_buffer (text);
-      gtk_box_append (GTK_BOX (container), child);
+      gtk_scrolled_window_set_child (GTK_SCROLLED_WINDOW (container), child);
       g_signal_connect (text, "changed",
                         G_CALLBACK (css_text_changed), provider);