syscall: fix duplicated copyright header in mksysnum_plan9.sh

Fixes #13569

Change-Id: I04c9a44437414177e2fee260518cfe53a67e6364
Reviewed-on: https://go-review.googlesource.com/17684Reviewed-by: Rob Pike <r@golang.org>

src/syscall/mksysnum_plan9.sh

 #!/bin/sh
 # Copyright 2009 The Go Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style
-# license that can be found in the LICENSE file.# Copyright 2009 The Go Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
 COMMAND="mksysnum_plan9.sh $@"