go.crypto/openpgp/packet: fix comment typo.

Thanks to Matt Goodall for pointing this out.

Change-Id: I30225299de2a6aba381f38bd753672c1137c3d5f
Reviewed-on: https://go-review.googlesource.com/11873
Reviewed-by: Adam Langley <agl@golang.org>
1 file changed