Replace modified_date with last_modified_at since jekyll-seo-tag would consider the later variable

Author: ngzhio

_layouts/post.html

@@ -12,9 +12,9 @@ <h1 class="post-title p-name" itemprop="name headline">{{ page.title | escape }}
       <time class="dt-published" datetime="{{ page.date | date_to_xmlschema }}" itemprop="datePublished">
         {{ page.date | date: date_format }}
       </time>
-      {%- if page.modified_date -%}
+      {%- if page.last_modified_at -%}
         ~ 
-        {%- assign mdate = page.modified_date | date_to_xmlschema -%}
+        {%- assign mdate = page.last_modified_at | date_to_xmlschema -%}
         <time class="dt-modified" datetime="{{ mdate }}" itemprop="dateModified">
           {{ mdate | date: date_format }}
         </time>