modify hackmd icon

lib/views/widget/page_tabs.html

@@ -18,7 +18,7 @@
   </li>
   <li class="nav-main-left-tab nav-tab-hackmd">
     <a {% if user %}href="#hackmd" data-toggle="tab"{% endif %} class="{% if not user %}edit-button-disabled{% endif %}">
-      <i class="icon-note"></i> {{ t('HackMD') }}
+      <i class="fa fa-file-text-o"></i> {{ t('HackMD') }}
     </a>
   </li>
   {% endif %}