[x/tour] go-tour: explain how to navigate

The extant instructions mention the Next button, which doesn't exist,
but not the table of contents menu, which does.

R=adg
CC=golang-dev
https://golang.org/cl/13673043
X-Tour-Commit: cbe3d607b688dde610f7147be2e4c129db70e763

tour/content/tour.article

@@ -34,7 +34,8 @@ These example programs demonstrate different aspects of Go. The programs in the
 
 Edit the program and run it again.
 
-Whenever you're ready to move on, click the Next button or type the PageDown key.
+When you're ready to move on, click the right arrow down below or type the PageDown key.
+You can also navigate using the menu under the "Go" flag at the top of the page.
 
 .play prog/tour/hello.go