Fix a command option name to be consistent with kubectl

[hidetatz]

kubectl-secret-data has a option --all-namespace, but it should be --all-namespaces because it is more consistent with kubectl. The difference of trailing "s" might confuse users who are used to the kubectl.

kubectl uses --all-namespaces like below:

$ kubectl get -h | grep all-namespace
 Prints a table of the most important information about the specified resources. You can filter the list using a label selector and the --selector flag. If the desired resource type is namespaced you will only see results in your current namespace unless you pass --all-namespaces.
  -A, --all-namespaces=false: If present, list the requested object(s) across all namespaces. Namespace in current context is ignored even if specified with --namespace.

[keisku]

Thank you for the contribution!