doc: revert copyright date to 2009

Somehow this date was changed in error (by me) to 2012.
It should have always been 2009.

Change-Id: I87029079458d4c4eeeff2f2fc0574f10afa9af09
Reviewed-on: https://go-review.googlesource.com/23622Reviewed-by: Rob Pike <r@golang.org>

LICENSE

-Copyright (c) 2012 The Go Authors. All rights reserved.
+Copyright (c) 2009 The Go Authors. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are