Update form_customization.rst

There are two code samples which are inconsistent which each other. I updated one of them.
minor #4689 Update form_customization.rst (rodrigorigotti)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4689).

Discussion
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Update form_customization.rst

This replaces a PR by @rodrigorigotti: https://github.com/symfony/symfony-docs/pull/4688. Many thanks to him!

Original PR description:

> There are two code samples which are inconsistent which each other. I updated one of them.

Commits
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4cd2046 Update form_customization.rst