minor symfony#11073 use correct code-block (OskarStark)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes symfony#11073).

Discussion
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use correct code-block

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Commits
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7ff8c9b use correct code-block

Author: javiereguiluz

workflow/usage.rst

@@ -413,7 +413,7 @@ of domain logic in your templates:
 
 The following example shows these functions in action:
 
-.. code-block:: twig
+.. code-block:: html+twig
 
     <h3>Actions</h3>
     {% if workflow_can(post, 'publish') %}