Fix "View this file on GitHub" links to work even when the default branch is not master

Author: kmyk

onlinejudge_verify_resources/_includes/document_header.html

@@ -3,7 +3,7 @@ <h1>{{ page.data._verificationStatusIcon }} {{ page.title }}
 </h1>
 
 <ul>
-    <li><a href="{{ site.github.repository_url }}/blob/master/{{ page.data.path }}">View this file on GitHub</a></li>
+    <li><a href="{{ site.github.repository_url }}/blob/{{ site.github.default_branch }}/{{ page.data.path }}">View this file on GitHub</a></li>
     <li>Last update: {{ page.data.timestamp }}</li>
     {% if page.data.attributes.PROBLEM %}
     <li>Problem: <a href="{{ page.data.attributes.PROBLEM }}">{{ page.data.attributes.PROBLEM }}</a></li>