fix etcd mixin new path

jsonnet/kube-prometheus/jsonnetfile.json

@@ -14,7 +14,7 @@
       "source": {
         "git": {
           "remote": "https://github.com/etcd-io/etcd",
-          "subdir": "Documentation/etcd-mixin"
+          "subdir": "contrib/mixin"
         }
       },
       "version": "master"

jsonnet/kube-prometheus/kube-prometheus-static-etcd.libsonnet

@@ -1,4 +1,4 @@
-(import 'github.com/etcd-io/etcd/Documentation/etcd-mixin/mixin.libsonnet') + {
+(import 'github.com/etcd-io/contrib/mixin/mixin.libsonnet') + {
   _config+:: {
     etcd: {
       ips: [],