update category_index.html

_includes/category_index.html

@@ -1,6 +1,6 @@
 <!-- <div class="category_index"> -->
 
-  {% assign items = site.posts | where: "group", "{{ page.title }}" %}
+  {% assign items = site.posts | where_exp: "item", "item.group == page.title" %}
 
   {% for item in items %}
   <div>