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 accountController the Response object is included, but not needed. Thirdly, the Request object is required in the code block for the method createAction in the accountController.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all (or 2.3+)
| Fixed tickets | none that I know of