Update navigation.html

The link for the Varnish book refers to the old one about Varnish 4. A new one has been released a couple of months ago which is about Varnish 6.

R1/source/_templates/navigation.html

@@ -23,7 +23,7 @@
         | <a href="{{ pathto("docs/6.0/index") }}">6.0 (supported)</a>
         | <a href="{{ pathto("docs/index") }}">other versions</a>
     </li>
-    <li class="toctree-l1"><a href="https://info.varnish-software.com/the-varnish-book">Varnish Book (PDF)</a></li>
+    <li class="toctree-l1"><a href="https://info.varnish-software.com/resources/varnish-6-by-example-book">Varnish Book (PDF)</a></li>
     <li class="toctree-l2">
 	<a href="http://book.varnish-software.com/4.0/">4.0</a>
         | <a href="http://book.varnish-software.com/3.0/">3.0</a>