Add missing itemprop for event description.

templates/mail_event.mustache

@@ -30,7 +30,7 @@ Content-Type: text/html; charset=utf-8
   </div>
   <div itemprop="reservationFor" itemscope itemtype="http://schema.org/{{reservationFor_type.value}}">
     Event: <strong><span itemprop="name">{{reservationFor_name.value}}</span></strong><br/>
-    {{reservationFor_description.value}}<br/>
+    <span itemprop="description">{{reservationFor_description.value}}</span><br/>
     (<a itemprop="url" href="{{reservationFor_url.value}}">{{reservationFor_url.value}}</a>)<br/>
     <link itemprop="image" href="{{reservationFor_image.value}}"/>
     <div itemprop="performer" itemscope itemtype="http://schema.org/Person">