Update form_customization.rst

In the Form chapter of Book for Symfony 2.3, we use `{{ form(form) }}` to render the entire form ( in contrast to `{{ form_widget(form) }}` for older version of Symfony ). so maybe it should use `{{ form(form) }}` here for consistency.
We need to also update the PHP version below with this same change.

And I think we should show both. The nice thing about `form(form)` is that it renders *everything*, including the `<form` tags. But, the disadvantage is that if you didn't add a submit button when building your form, you can't add it here. So, I think both are useful. How about:

```jinja
{# renders all of the fields #}
{{ form_widget(form) }}

{# renders all of the fields *and* the <form> start and end tag #}
{{ form(form) }}
```

What do you think?