Fix the name of the Let's Encrypt cert execute resource

The resource in the notification was invalid, missing the type of
resource (execute)

Fixes #171

site-cookbooks/kosmos-dirsrv/resources/instance.rb

@@ -157,7 +157,7 @@ done
     end
 
     nginx_certbot_site new_resource.hostname do
-      notifies :run, "letsencrypt cert for #{domain}", :delayed
+      notifies :run, "execute[letsencrypt cert for #{domain}]", :delayed
     end
 
     # Generate a Let's Encrypt cert (only if the nginx vhost exists and no cert