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 and use `h6` if three of them already do.)