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/share-process-namespace.yaml

@@ -8,7 +8,7 @@ spec:
   - name: nginx
     image: nginx
   - name: shell
-    image: busybox
+    image: busybox:1.28
     securityContext:
       capabilities:
         add: