Updated documentation to use beta version of init-containers

docs/user-guide/nginx-init-containers.yaml

@@ -3,7 +3,7 @@ kind: Pod
 metadata:
   name: nginx
   annotations:
-    pod.alpha.kubernetes.io/init-containers: '[
+    pod.beta.kubernetes.io/init-containers: '[
         {
             "name": "install",
             "image": "busybox",

docs/user-guide/petset/bootstrapping/petset_peers.yaml

@@ -29,7 +29,7 @@ spec:
         app: nginx
       annotations:
         pod.alpha.kubernetes.io/initialized: "true"
-        pod.alpha.kubernetes.io/init-containers: '[
+        pod.beta.kubernetes.io/init-containers: '[
             {
                 "name": "peerfinder",
                 "image": "gcr.io/google_containers/peer-finder:0.1",

docs/user-guide/petset/bootstrapping/petset_vm.yaml

@@ -27,7 +27,7 @@ spec:
         app: ub
       annotations:
         pod.alpha.kubernetes.io/initialized: "true"
-        pod.alpha.kubernetes.io/init-containers: '[
+        pod.beta.kubernetes.io/init-containers: '[
             {
                 "name": "rootfs",
                 "image": "ubuntu:15.10",