chacha20poly1305: fix typos

Change-Id: I55a2ad4495f4e1164af6a8504b035cf658f8b822
Reviewed-on: https://go-review.googlesource.com/34536
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed