Linting fix to adhere to curlylint 0.13

wagtail/admin/templates/wagtailadmin/shared/side_panels/preview.html

@@ -57,7 +57,7 @@
         <div class="preview-panel__wrapper">
             <iframe loading="lazy" title="{% trans 'Preview' %}" class="preview-panel__iframe" data-preview-iframe aria-describedby="preview-panel-error-banner">
                 <div>
-                     {# Add placeholder element to support styling content when iframe has loaded #}
+                    {# Add placeholder element to support styling content when iframe has loaded #}
                 </div>
             </iframe>
         </div>