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

This redundancy is now caught by the improved printf vet checker
(#30436).

Updates #49322

Change-Id: Id450247adc6fa28a9244c019be3c1b52c2d17f49
Reviewed-on: https://go-review.googlesource.com/c/go/+/361263
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Zvonimir Pavlinovic <zpavlinovic@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

src/cmd/cover/cover.go

@@ -40,8 +40,8 @@ Finally, to generate modified source code with coverage annotations
 `
 
 func usage() {
-	fmt.Fprintln(os.Stderr, usageMessage)
-	fmt.Fprintln(os.Stderr, "Flags:")
+	fmt.Fprint(os.Stderr, usageMessage)
+	fmt.Fprintln(os.Stderr, "\nFlags:")
 	flag.PrintDefaults()
 	fmt.Fprintln(os.Stderr, "\n  Only one of -html, -func, or -mode may be set.")
 	os.Exit(2)