fix variable name

Author: fmichonneau

_includes/navbar.html

@@ -49,7 +49,7 @@
         <li><a href="{{ relative_root_path }}{% link setup.md %}">Setup</a></li>
 
         {% comment %} Show lesson episodes for lessons. {% endcomment %}
-        {% if lesson_episodes.length > 0 %}
+        {% if lesson_episodes.size > 0 %}
         <li class="dropdown">
           <a href="{{ relative_root_path }}/" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Episodes <span class="caret"></span></a>
           <ul class="dropdown-menu">