net: fix format string in TestAcceptIgnoreSomeErrors

LGTM=mikioh.mikioh
R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/84340043

src/pkg/net/net_windows_test.go

@@ -84,7 +84,7 @@ func TestAcceptIgnoreSomeErrors(t *testing.T) {
 	}
 	err = cmd.Start()
 	if err != nil {
-		t.Fatalf("cmd.Start failed: %v\n%s\n", err)
+		t.Fatalf("cmd.Start failed: %v\n", err)
 	}
 	outReader := bufio.NewReader(stdout)
 	for {