Update etcd installation instructions (kubernetes#7010)

* Update etcd installation instructions
* Add link referencing the etcd.manifest file

Author: cimomo

docs/getting-started-guides/scratch.md

@@ -537,7 +537,7 @@ availability.
 
 To run an etcd instance:
 
-1. Copy `cluster/saltbase/salt/etcd/etcd.manifest`
+1. Copy [`cluster/gce/manifests/etcd.manifest`](https://github.com/kubernetes/kubernetes/blob/master/cluster/gce/manifests/etcd.manifest)
 1. Make any modifications needed
 1. Start the pod by putting it into the kubelet manifest directory