minor #9277 [Fix] Custom FormType namespace (orions)

This PR was submitted for the master branch but it was merged into the 4.0 branch instead (closes #9277).

Discussion
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[Fix] Custom FormType namespace

Hello,

With SF4 we can build bundle-less apps so `FormType` should not be in `<BundleName>\Form\Type`.

best.

Commits
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492e81a [Fix] Custom FormType namespace

Author: javiereguiluz

form/create_custom_field_type.rst

@@ -16,7 +16,7 @@ Defining the Field Type
 In order to create the custom field type, first you have to create the class
 representing the field. In this situation the class holding the field type
 will be called ``ShippingType`` and the file will be stored in the default location
-for form fields, which is ``<BundleName>\Form\Type``. Make sure the field extends
+for form fields, which is ``App\Form\Type``. Make sure the field extends
 :class:`Symfony\\Component\\Form\\AbstractType`::
 
     // src/Form/Type/ShippingType.php