update designate tempest link

The tempest guideline link require an update.

Change-Id: If993d362d48cf54829a4bcca7e53e1f41d082636
diff --git a/README.rst b/README.rst
index 09d8191..30743cf 100644
--- a/README.rst
+++ b/README.rst
@@ -18,7 +18,7 @@
 http://docs.openstack.org/developer/tempest/plugin.html#using-plugins
 
 See the designate docs for information on writing new tests etc:
-http://docs.openstack.org/developer/designate/tempest.html
+https://docs.openstack.org/developer/designate-tempest-plugin/#writing-new-tests
 
 Running the tests
 -----------------