go-tour/gotour: fix bug in argument handling

Don't use the default after going to all that trouble to override it.
Mea culpa.

R=adg
CC=golang-dev
https://codereview.appspot.com/13638043

gotour/tour.go

@@ -35,7 +35,7 @@ func initTour(root string) error {
 	f, err := os.Open(source)
 	if err != nil {
 		// See if it exists in the content directory in the root.
-		source = filepath.Join(root, "content", "tour.article")
+		source = filepath.Join(root, "content", source)
 		f, err = os.Open(source)
 		if err != nil {
 			return err