http: make triv.go example compile again

Ideally we'd compile all example files during
the build, though.

Fixes #1660

R=r
CC=golang-dev
https://golang.org/cl/4358049

src/pkg/http/triv.go

@@ -56,7 +56,7 @@ func (ctr *Counter) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 var booleanflag = flag.Bool("boolean", true, "another flag for testing")
 
 func FlagServer(w http.ResponseWriter, req *http.Request) {
-	w.Header.Set("Content-Type", "text/plain; charset=utf-8")
+	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
 	fmt.Fprint(w, "Flags:\n")
 	flag.VisitAll(func(f *flag.Flag) {
 		if f.Value.String() != f.DefValue {