Fix download link in aws/kubecfg documentation

Kubecfg's download link on Darwin and Linux are points to http://storage.googleapis.com/k8s/darwin/kubecfg .
Later one should be changed to http://storage.googleapis.com/k8s/linux/kubecfg .
Merge pull request #2414 from hodduc/fix-aws-doc

Fix download link in aws/kubecfg documentation