Fix help text for runnable plugin command

When creating a plugin without sub commands, the help text included the
command name (kubectl-plugin) instead of the display name (kubectl plugin):

    Usage:
      kubectl-plugin [flags]

The issue is that the usage line for this case does not use the
command path but the raw `Use` string, and this case was not tested.

Add a test for this case and fix UsageLine() to replace the command name
with the display name.

Tested using https://github.com/nirs/kubernetes/tree/sample-cli-plugin-help

command.go

@@ -1441,10 +1441,11 @@ func (c *Command) displayName() string {
 // UseLine puts out the full usage for a given command (including parents).
 func (c *Command) UseLine() string {
 	var useline string
+	use := strings.Replace(c.Use, c.Name(), c.displayName(), 1)
 	if c.HasParent() {
-		useline = c.parent.CommandPath() + " " + c.Use
+		useline = c.parent.CommandPath() + " " + use
 	} else {
-		useline = c.Use
+		useline = use
 	}
 	if c.DisableFlagsInUseLine {
 		return useline

command_test.go

@@ -370,6 +370,26 @@ func TestAliasPrefixMatching(t *testing.T) {
 // executable is `kubectl-plugin`, but we run it as `kubectl plugin`. The help
 // text should reflect the way we run the command.
 func TestPlugin(t *testing.T) {
+	cmd := &Command{
+		Use:  "kubectl-plugin",
+		Args: NoArgs,
+		Annotations: map[string]string{
+			CommandDisplayNameAnnotation: "kubectl plugin",
+		},
+		Run: emptyRun,
+	}
+
+	cmdHelp, err := executeCommand(cmd, "-h")
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	checkStringContains(t, cmdHelp, "kubectl plugin [flags]")
+	checkStringContains(t, cmdHelp, "help for kubectl plugin")
+}
+
+// TestPlugin checks usage as plugin with sub commands.
+func TestPluginWithSubCommands(t *testing.T) {
 	rootCmd := &Command{
 		Use:  "kubectl-plugin",
 		Args: NoArgs,