Update form_customization.rst

In the Form chapter of Book for Symphony 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.
Applied suggestion by Ryan
minor #4399 Explain form() and form_widget() in form customization (oopsFrogs, WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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Explain form() and form_widget() in form customization

Continues https://github.com/symfony/symfony-docs/pull/4365

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed tickets | -

Original PR description:

> 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.

Commits
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0086c5e Applied suggestion by Ryan
d653ff4 Update form_customization.rst