http2: use updated URI in doc

Fixes golang/go#33246

Change-Id: Iefb39f086eebfa2707725f7464339fca6cff9afd
Reviewed-on: https://go-review.googlesource.com/c/net/+/187277
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

http2/transport.go

@@ -992,7 +992,7 @@ func (cc *ClientConn) roundTrip(req *http.Request) (res *http.Response, gotErrAf
 		req.Method != "HEAD" {
 		// Request gzip only, not deflate. Deflate is ambiguous and
 		// not as universally supported anyway.
-		// See: http://www.gzip.org/zlib/zlib_faq.html#faq38
+		// See: https://zlib.net/zlib_faq.html#faq39
 		//
 		// Note that we don't request this for HEAD requests,
 		// due to a bug in nginx: