Fix broken link in doc
Fix broken link for vnffgd_template_description.rst in
vnffg_usage_guide.rst file.
@@ -25,7 +25,7 @@ decides which traffic should go through them.
Similar to how VNFs are described by VNFDs, VNFFGs are described by VNF
Forwarding Graph Descriptors (VNFFGD). Please see the `devref guide
-<https://github.com/openstack/tacker/tree/doc/source/devref
+<https://github.com/openstack/tacker/blob/master/doc/source/devref
/vnffgd_template_description.rst>`_ on VNFFGD to learn more about
how a VNFFGD is defined.
