Fix previews grouping with last categorie

Jellyfin.Plugin.SkinManager/Configuration/configurationpage.html

@@ -101,6 +101,7 @@
         return "";
       }
       var html = "";
+      html += '</fieldset>';
       html += '<h2 class="sectionTitle" >Previews</h2>';
         skin.previews.forEach(element => {          
            html += getImage(element.url);