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/security/hello-apparmor.yaml

@@ -9,5 +9,5 @@ metadata:
 spec:
   containers:
   - name: hello
-    image: busybox
+    image: busybox:1.28
     command: [ "sh", "-c", "echo 'Hello AppArmor!' && sleep 1h" ]

content/en/examples/pods/security/security-context.yaml

@@ -12,7 +12,7 @@ spec:
     emptyDir: {}
   containers:
   - name: sec-ctx-demo
-    image: busybox
+    image: busybox:1.28
     command: [ "sh", "-c", "sleep 1h" ]
     volumeMounts:
     - name: sec-ctx-vol