minor #9581 Fix "payload" usage (liviubalan)

This PR was submitted for the 3.4 branch but it was merged into the 2.7 branch instead (closes #9581).

Discussion
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Fix "payload" usage

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Commits
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f69f68f Fix "payload" usage

Author: javiereguiluz

validation/severity.rst

@@ -155,8 +155,8 @@ so that the severity is added as an additional HTML class:
         {%- if errors|length > 0 -%}
         <ul>
             {%- for error in errors -%}
-                {% if error.cause.constraint.payload.severity is defined %}
-                    {% set severity = error.cause.constraint.payload.severity %}
+                {% 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>
             {%- endfor -%}