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/docs/concepts/workloads/pods/_index.md

@@ -180,7 +180,7 @@ spec:
     spec:
       containers:
       - name: hello
-        image: busybox
+        image: busybox:1.28
         command: ['sh', '-c', 'echo "Hello, Kubernetes!" && sleep 3600']
       restartPolicy: OnFailure
     # The pod template ends here