http2/h2demo: update gopher image URL

The URL of the go-programming-language-turns-two_gophers.jpg image
has changed to https://blog.golang.org/2years/2years-gophers.jpg
in CL 223599. Update it here accordingly.

Fixes golang/go#37976

Change-Id: I43d5ee79e0049700bf91a266f0e23b48b3ce53f9
Reviewed-on: https://go-review.googlesource.com/c/net/+/224578
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

http2/h2demo/h2demo.go

@@ -323,7 +323,7 @@ func newPushHandler() http.Handler {
 }
 
 func newGopherTilesHandler() http.Handler {
-	const gopherURL = "https://blog.golang.org/go-programming-language-turns-two_gophers.jpg"
+	const gopherURL = "https://blog.golang.org/2years/2years-gophers.jpg"
 	res, err := http.Get(gopherURL)
 	if err != nil {
 		log.Fatal(err)