networkAgent: Fix showing the WPS message

The message was introduced in commit dfa0750ffd, but added to a
non-existent container, whoops.

Spotted by eslint.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/375

js/ui/components/networkAgent.js

@@ -111,7 +111,7 @@ var NetworkSecretDialog = class extends ModalDialog.ModalDialog {
             descriptionLabel.clutter_text.line_wrap = true;
             descriptionLabel.clutter_text.ellipsize = Pango.EllipsizeMode.NONE;
 
-            messageBox.add(descriptionLabel,
+            contentBox.messageBox.add(descriptionLabel,
                                       { y_fill:  true,
                                         y_align: St.Align.START,
                                         expand: true });