docs: improve k8s usage documentation (#2425)

Signed-off-by: Jose Donizetti <jdbjunior@gmail.com>

pkg/commands/app.go

@@ -862,6 +862,7 @@ func NewK8sCommand() *cli.Command {
 		Name:      "kubernetes",
 		Aliases:   []string{"k8s"},
 		Usage:     "scan kubernetes vulnerabilities, secrets and misconfigurations",
+		UsageText: "trivy k8s [command options] <cluster|all| or specific resources like kubectl. eg: pods, pod/NAME>",
 		CustomHelpTemplate: cli.CommandHelpTemplate + `EXAMPLES:
   - cluster scanning:
       $ trivy k8s --report summary cluster