Translate "helpers" to JA

_includes/workshop_ad.html

@@ -39,7 +39,7 @@ <h2>{{page.venue}}</h2>
           </p>
           {% if page.helper %}
           <p>
-          <strong>Helpers:</strong>
+          <strong>ヘルパー:</strong>
           {{page.helper | join: ', ' %}}
           </p>
           {% endif %}