net/http: fix triv.go

net/http: fix triv.go

CL 428137 replaced 'buf := make(bytes.Buffer)' with 'var buf strings.Builder'.
That change also requires passing &buf to io.Copy instead of buf.

Change-Id: I72b3faa46693e7d1441298f49dc6b95859c3bff3
Reviewed-on: https://go-review.googlesource.com/c/go/+/449635
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>