cloud/storage/s3: add missing copyright header
cloud/storage/s3: add missing copyright header

Change-Id: Ic4b6177d35783af1903b29efb244ce9a52a45d5d
Reviewed-on: https://upspin-review.googlesource.com/16620
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/aws