Fix link to SkyDNS (#637)

Url was wrong

middleware/etcd/README.md

@@ -2,7 +2,7 @@
 
 *etcd* enables reading zone data from an etcd instance. The data in etcd has to be encoded as
 a [message](https://github.com/skynetservices/skydns/blob/2fcff74cdc9f9a7dd64189a447ef27ac354b725f/msg/service.go#L26)
-like [SkyDNS](https//github.com/skynetservices/skydns). It should also work just like SkyDNS.
+like [SkyDNS](https://github.com/skynetservices/skydns). It should also work just like SkyDNS.
 
 The etcd middleware makes extensive use of the proxy middleware to forward and query other servers
 in the network.