GSettings: Don't mention nonexistent signal in doc

https://bugzilla.gnome.org/show_bug.cgi?id=622126

gio/gsettings.c

@@ -2475,8 +2475,7 @@ g_settings_list_keys (GSettings *settings)
  * to call g_settings_get_child().
  *
  * For GSettings objects that are lists, this value can change at any
- * time and you should connect to the "children-changed" signal to watch
- * for those changes.  Note that there is a race condition here: you may
+ * time. Note that there is a race condition here: you may
  * request a child after listing it only for it to have been destroyed
  * in the meantime.  For this reason, g_settings_get_child() may return
  * %NULL even for a child that was listed by this function.