Make the box.net configuration instructions clearer

When you're configuring a box.net repository, Moodle provides some guidance on what you need to do in box.net. Step 2 says...

Make sure you set the callback URL of this box.net service to (URL)

The supplied URL appears to need to go in "Redirect URL" under "backend parameters". There's no callback URL field available on the box.net application configuration page.