minor #9603 Updated a Twig example (javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
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Updated a Twig example

After merging #9581 I realized that we could improve the example using modern Twig code.

Commits
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b1659f1 Updated a Twig example

Author: javiereguiluz

validation/severity.rst

@@ -155,10 +155,7 @@ so that the severity is added as an additional HTML class:
         {%- if errors|length > 0 -%}
         <ul>
             {%- for error in errors -%}
-                {% if error.constraint.payload.severity is defined %}
-                    {% set severity = error.constraint.payload.severity %}
-                {% endif %}
-                <li{% if severity is defined %} class="{{ severity }}"{% endif %}>{{ error.message }}</li>
+                <li class="{{ error.constraint.payload.severity ?? '' }}">{{ error.message }}</li>
             {%- endfor -%}
         </ul>
         {%- endif -%}