cmd/trace: use fmt.Print for newline-ending fixed string

This redundancy is now caught by the improved printf vet checker.

Updates #49322

Change-Id: Ic7a931b8d4838be02ebb855b69624b95093bd114
Reviewed-on: https://go-review.googlesource.com/c/go/+/361265
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Zvonimir Pavlinovic <zpavlinovic@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/trace/main.go

@@ -66,7 +66,7 @@ var (
 
 func main() {
 	flag.Usage = func() {
-		fmt.Fprintln(os.Stderr, usageMessage)
+		fmt.Fprint(os.Stderr, usageMessage)
 		os.Exit(2)
 	}
 	flag.Parse()