net/http: fix comment

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6348062

src/pkg/net/http/serve_test.go

@@ -1270,7 +1270,7 @@ func BenchmarkClientServer(b *testing.B) {
 //
 // For use like:
 //   $ go test -c
-//   $ ./http.test -test.run=XX -test.bench=Benchmarktime=15 -test.cpuprofile=http.prof
+//   $ ./http.test -test.run=XX -test.bench=BenchmarkServer -test.benchtime=15 -test.cpuprofile=http.prof
 //   $ go tool pprof http.test http.prof
 //   (pprof) web
 func BenchmarkServer(b *testing.B) {