README: update TextUnmarshaler example

Values are much more convenient to use in argument structs, so update
README to use them instead of pointers in the example as we now support
this.

Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>

README.md

@@ -288,10 +288,10 @@ func (n *NameDotName) MarshalText() (text []byte, err error) {
 
 func main() {
 	var args struct {
-		Name *NameDotName
+		Name NameDotName
 	}
 	// set default
-	args.Name = &NameDotName{"file", "txt"}
+	args.Name = NameDotName{"file", "txt"}
 	arg.MustParse(&args)
 	fmt.Printf("%#v\n", args.Name)
 }
@@ -305,10 +305,10 @@ Options:
   --help, -h             display this help and exit
 
 $ ./example
-&main.NameDotName{Head:"file", Tail:"txt"}
+main.NameDotName{Head:"file", Tail:"txt"}
 
 $ ./example --name=foo.bar
-&main.NameDotName{Head:"foo", Tail:"bar"}
+main.NameDotName{Head:"foo", Tail:"bar"}
 
 $ ./example --name=oops
 Usage: example [--name NAME]