minor symfony#6409 Update 'date' to DateType::class in form.rst (iltar)

This PR was merged into the 2.8 branch.

Discussion
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Update 'date' to DateType::class in form.rst

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

A small patch for a small issue. This patch should be merged upwards as 3.0 also has the old notation.

Commits
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23abecc Update 'date' to DateType::class in form.rst

book/forms.rst

@@ -694,7 +694,7 @@ the documentation for each type.
     :ref:`disable HTML5 validation <book-forms-html5-validation-disable>`
     or set the ``required`` option on your field to ``false``::
     
-        ->add('dueDate', 'date', array(
+        ->add('dueDate', DateType::class, array(
             'widget' => 'single_text',
             'required' => false
         ))