plan9: fix duplicated copyright header in mksysnum_plan9.sh

Fixes golang/go#13569.

Change-Id: I8af36bc81afa6b8bc0e80b3f35f0b60707bfb91b
Reviewed-on: https://go-review.googlesource.com/17713Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

plan9/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 $@"