minor #15248 [minor] Use correct code block (Nyholm)

This PR was submitted for the 5.x branch but it was merged into the 4.4 branch instead.

Discussion
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[minor] Use correct code block

We use `html+php` on all other places with short open syntax.

Commits
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28d79d6 [minor] Use correct code block

Author: javiereguiluz

reference/forms/types/options/preferred_choices.rst.inc

@@ -64,7 +64,7 @@ when rendering the field:
 
         {{ form_widget(form.publishAt, { 'separator': '=====' }) }}
 
-    .. code-block:: php
+    .. code-block:: html+php
 
         <?= $view['form']->widget($form['publishAt'], [
             'separator' => '=====',