http2/h2demo: use HTTP host for HTTP-only link

Change-Id: Ic749ab27f45472b9a21dee9a16567d2f7741d9b1
Reviewed-on: https://go-review.googlesource.com/38780
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

http2/h2demo/tmpl.go

@@ -43,7 +43,7 @@ Note: This page exists for demonstration purposes. For the actual cmd/go docs, g
 <div style="padding:20px">
 
 
-<a href="https://{{.HTTPSHost}}/serverpush">HTTP/2 with Server Push</a> | <a href="http://{{.HTTPSHost}}/serverpush">HTTP only</a>
+<a href="https://{{.HTTPSHost}}/serverpush">HTTP/2 with Server Push</a> | <a href="http://{{.HTTPHost}}/serverpush">HTTP only</a>
 <div id="loadtimes"></div>
 
 </div>