Update edit.html.erb

I don't have an aesthetic opinion one way or another, but for consistency's sake -- e.g., with https://github.com/spree/spree_auth_devise/blob/2-0-stable/app/views/spree/user_passwords/new.html.erb as well as with the [login](https://github.com/spree/spree_auth_devise/blob/2-0-stable/app/views/spree/user_sessions/new.html.erb) and [signup](https://github.com/spree/spree_auth_devise/blob/2-0-stable/app/views/spree/user_registrations/new.html.erb) views, I reckon we should wrap the relevant lines in some kind of div and use h6 rather than h2.

(Although in my personal opinion we should use h2 for all of these four, but I'm happy to go with the convention.)