net/http: add example for FileServer to mention StripPrefix

Fixes #3530.

R=golang-dev, rsc, bradfitz
CC=golang-dev
https://golang.org/cl/6032052

src/pkg/net/http/example_test.go

@@ -49,3 +49,8 @@ func ExampleGet() {
 	res.Body.Close()
 	fmt.Printf("%s", robots)
 }
+
+func ExampleFileServer() {
+	// we use StripPrefix so that /tmpfiles/somefile will access /tmp/somefile
+	http.Handle("/tmpfiles/", http.StripPrefix("/tmpfiles/", http.FileServer(http.Dir("/tmp"))))
+}