http: update triv.go with gofix

Sample code in triv.go wouldn't compile because it didn't use the new
FileSystem interface.
This was discussed on golang-nuts on July 7.
https://groups.google.com/forum/#!topic/golang-nuts/NMhZk3Ft_Vc

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/4684044

src/pkg/http/triv.go

@@ -138,7 +138,7 @@ func main() {
 	expvar.Publish("counter", ctr)
 
 	http.Handle("/", http.HandlerFunc(Logger))
-	http.Handle("/go/", http.FileServer(*webroot, "/go/"))
+	http.Handle("/go/", http.StripPrefix("/go/", http.FileServer(http.Dir(*webroot))))
 	http.Handle("/flags", http.HandlerFunc(FlagServer))
 	http.Handle("/args", http.HandlerFunc(ArgServer))
 	http.Handle("/go/hello", http.HandlerFunc(HelloServer))