Fix name of file.

ticdesk/templates/503.html

@@ -24,7 +24,7 @@
    </p>
   </div>
   <div class="col-xs-12 col-md-3">
-   <img src="{% static 'maintenance-dolphin.png' %}"
+   <img src="{% static 'maintenance_dolphin.png' %}"
         alt="Maybe something new will be developed…" class="full-width" />
   </div>
  </div>