[Website] Update outdated link in top nav to columnar specification

diff --git a/_includes/header.html b/_includes/header.html
index 4174bae..fc4df47 100644
--- a/_includes/header.html
+++ b/_includes/header.html
@@ -37,7 +37,7 @@
           </div>
         </li>
         <li class="nav-item">
-          <a class="nav-link" href="{{ site.baseurl }}/docs/format/README.html"
+          <a class="nav-link" href="{{ site.baseurl }}/docs/format/Columnar.html"
              role="button" aria-haspopup="true" aria-expanded="false">
              Specification
           </a>