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: #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

Author: wouterj

cookbook/form/form_customization.rst

@@ -502,7 +502,8 @@ resource to use such a layout:
         # app/config/config.yml
         twig:
             form:
-                resources: ['form_table_layout.html.twig']
+                resources:
+                    - 'form_table_layout.html.twig'
             # ...
 
     .. code-block:: xml