Fixes to cookbook/doctrine/registration_form.rst

As I was working through this example I noticed a few issues. Firstly, no button to submit the registration form is ever specified. Secondly, in the Account Controller the Response object is included, but not needed. It is the Request object that is required.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all (or 2.3+)
| Fixed tickets | none that I know of