droptarget: Improve the docs

Add the signal connection to the example.

gtk/gtkdroptarget.c

@@ -80,6 +80,7 @@
  *     GDK_TYPE_PIXBUF,
  *   }, 2);
  *
+ *   g_signal_connect (target, "drop", G_CALLBACK (on_drop), self);
  *   gtk_widget_add_controller (GTK_WIDGET (self), GTK_EVENT_CONTROLLER (target));
  * }
  * ```