net/http: fix error message in TestClientWriteShutdown

Change-Id: I3c664201baef6d7dbed94dab63db0ac974bf6817
Reviewed-on: https://go-review.googlesource.com/33198
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/serve_test.go

@@ -2470,7 +2470,7 @@ func TestClientWriteShutdown(t *testing.T) {
 	}
 	err = conn.(*net.TCPConn).CloseWrite()
 	if err != nil {
-		t.Fatalf("Dial: %v", err)
+		t.Fatalf("CloseWrite: %v", err)
 	}
 	donec := make(chan bool)
 	go func() {