ARROW-6246: [Website] Add link to R documentation site

diff --git a/_includes/header.html b/_includes/header.html
index 7c02533..4174bae 100644
--- a/_includes/header.html
+++ b/_includes/header.html
@@ -52,9 +52,10 @@
             <a class="dropdown-item" href="{{ site.baseurl }}/docs">Project Docs</a>
             <a class="dropdown-item" href="{{ site.baseurl }}/docs/python">Python</a>
             <a class="dropdown-item" href="{{ site.baseurl }}/docs/cpp">C++</a>
-            <a class="dropdown-item" href="{{ site.baseurl }}/docs/java">Java API</a>
-            <a class="dropdown-item" href="{{ site.baseurl }}/docs/c_glib">C GLib API</a>
-            <a class="dropdown-item" href="{{ site.baseurl }}/docs/js">Javascript API</a>
+            <a class="dropdown-item" href="{{ site.baseurl }}/docs/java">Java</a>
+            <a class="dropdown-item" href="{{ site.baseurl }}/docs/c_glib">C GLib</a>
+            <a class="dropdown-item" href="{{ site.baseurl }}/docs/js">JavaScript</a>
+            <a class="dropdown-item" href="{{ site.baseurl }}/docs/r">R</a>
           </div>
         </li>
         <!-- <li><a href="{{ site.baseurl }}/blog">Blog</a></li> -->