fix busybox image to 1.28 (issues with nslookup).

Changes where done with these commands:

reprec 'image: busybox(?!:)' 'image: busybox:1.28' */docs */examples
reprec -- '--image=busybox(?!:)' '--image=busybox:1.28' */docs */examples

Related issues:

 https://github.com/docker-library/busybox/issues/48
 https://github.com/kubernetes/kubernetes/issues/66924

content/en/examples/pods/init-containers.yaml

@@ -14,7 +14,7 @@ spec:
   # These containers are run during pod initialization
   initContainers:
   - name: install
-    image: busybox
+    image: busybox:1.28
     command:
     - wget
     - "-O"