readme: use golang.org links rather than github.com

Change-Id: I2b0b87aed4ff5c338b0ae09b63f55411322a871e
Reviewed-on: https://go-review.googlesource.com/15436
Reviewed-by: Andrew Gerrand <adg@golang.org>

http2/README

@@ -17,4 +17,4 @@ Demo test server at https://http2.golang.org/
 Help & bug reports welcome!
 
 Contributing: https://golang.org/doc/contribute.html
-Bugs:         https://github.com/golang/go/issues/new?title=x/net/http2:+
+Bugs:         https://golang.org/issue/new?title=x/net/http2:+