net/http: fix a nit

Change-Id: I31fa5f906ad2e8dc475dbbeb91f568f91e16861b
Reviewed-on: https://go-review.googlesource.com/35514
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/serve_test.go

@@ -5277,7 +5277,7 @@ func TestServerHijackGetsBackgroundByte_big(t *testing.T) {
 		defer conn.Close()
 		slurp, err := ioutil.ReadAll(buf.Reader)
 		if err != nil {
-			t.Error("Copy: %v", err)
+			t.Errorf("Copy: %v", err)
 		}
 		allX := true
 		for _, v := range slurp {