Fix kubectl-ns plugin help text

The plugin help text was not consistent - using `ns` in some cases and
`kubectl ns` in others.

With this change we always have `kubectl ns`:

    $ kubectl ns -h
    View or set the current namespace

    Usage:
      kubectl ns [new-namespace] [flags]

    Examples:

            # view the current namespace in your KUBECONFIG
            kubectl ns

            # view all of the namespaces in use by contexts in your KUBECONFIG
            kubectl ns --list

            # switch your current-context to one that contains the desired namespace
            kubectl ns foo

    Flags:
      ...
      -h, --help                           help for kubectl ns

staging/src/k8s.io/sample-cli-plugin/pkg/cmd/ns.go

@@ -82,6 +82,9 @@ func NewCmdNamespace(streams genericiooptions.IOStreams) *cobra.Command {
 		Short:        "View or set the current namespace",
 		Example:      fmt.Sprintf(namespaceExample, "kubectl"),
 		SilenceUsage: true,
+		Annotations: map[string]string{
+			cobra.CommandDisplayNameAnnotation: "kubectl ns",
+		},
 		RunE: func(c *cobra.Command, args []string) error {
 			if err := o.Complete(c, args); err != nil {
 				return err