minor #11383 use correct code block for examples (OskarStark)

This PR was merged into the 4.2 branch.

Discussion
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use correct code block for examples

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Commits
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f4031b9 use correct code block for examples

Author: javiereguiluz

form/form_customization.rst

@@ -30,7 +30,7 @@ The next step is to use the :ref:`form_start() <reference-forms-twig-start>`,
 :ref:`form_row() <reference-forms-twig-row>` Twig functions to render the
 different form parts so you can customize them adding HTML elements and attributes:
 
-.. code-block:: twig
+.. code-block:: html+twig
 
     {{ form_start(form) }}
         <div class="my-custom-class-for-errors">

form/form_themes.rst

@@ -122,7 +122,7 @@ A form can also be customized by applying several themes. To do this, pass the
 path of all the Twig templates as an array using the ``with`` keyword (their
 order is important, because each theme overrides all the previous ones):
 
-.. code-block:: html+twig
+.. code-block:: twig
 
     {# apply multiple form themes but only to the form of this template #}
     {% form_theme form with [