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>

src/net/http/triv.go

@@ -50,7 +50,7 @@ func (ctr *Counter) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 		ctr.n++
 	case "POST":
 		var buf strings.Builder
-		io.Copy(buf, req.Body)
+		io.Copy(&buf, req.Body)
 		body := buf.String()
 		if n, err := strconv.Atoi(body); err != nil {
 			fmt.Fprintf(w, "bad POST: %v\nbody: [%v]\n", err, body)