doc: added example of InvalidOption (#44)

doc.go

@@ -247,11 +247,11 @@ And help text can be generated as follows:
 	// (stdout)
 	// This is the usage of this command.
 	//
-	// OPTIONS
+	// OPTIONS:
 	//   --foo     The description of foo option.
 	//   ...
 	//
-	// SUB COMMANDS
+	// SUB COMMANDS:
 	// list        The description of list sub-command.
 	//   --bar     The description of bar option.
 	//   ...

example_invalid-option_test.go

@@ -0,0 +1,28 @@
+package cliargs_test
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/sttk/cliargs"
+)
+
+func ExampleInvalidOption() {
+	optCfgs := []cliargs.OptCfg{
+		cliargs.OptCfg{
+			Name:    "foo",
+			Default: []string{"123"},
+			HasArg:  false,
+		},
+	}
+
+	_, e := cliargs.ParseWith(os.Args, optCfgs)
+	ee := e.(cliargs.InvalidOption)
+
+	fmt.Printf("error type: %T\n", ee)
+	fmt.Printf("option: %s\n", ee.GetOpt())
+
+	// Output:
+	// error type: cliargs.ConfigHasDefaultButHasNoArg
+	// option: foo
+}