bug #6086 Update form_customization.rst (vudaltsov)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes #6086).

Discussion
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Update form_customization.rst

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | >= 2.8
| Fixed tickets |

Commits
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36d9470 Update form_customization.rst

cookbook/form/form_customization.rst

@@ -101,7 +101,7 @@ rendering a form. In other words, if you want to customize one portion of
 how a form is rendered, you'll import a *theme* which contains a customization
 of the appropriate form fragments.
 
-Symfony comes with four **built-in form themes** that define each and every
+Symfony comes with five **built-in form themes** that define each and every
 fragment needed to render every part of a form:
 
 * `form_div_layout.html.twig`_, wraps each form field inside a ``<div>`` element.