The registration page says:
but there's no way to do this yet, which I've seen frustrate some new users. There's also no indication once you have an account that you might be able to change it by emailing support.
(This is also referred to as a page "handle" elsewhere on the site, I think? on the new page creation screen?)
This really needs to be an option available to people, especially when it's stated at registration that you can change it.
This is now live! Visit your page settings to change it.