Update header.html to change blog link to Medium

Author: rjurney

_includes/header.html

@@ -6,7 +6,7 @@
 
     <nav class="custom-nav">
       <div class="trigger show-link">
-        <a class="page-link" href="/blog">Blog</a>
+        <a class="page-link" href="https://blog.graphlet.ai">Blog</a>
         <a
           class="page-link"
           href="https://github.com/Graphlet-AI/graphlet"