chacha20poly1305: clarify the correct attribution for the AMD64 assembly.
chacha20poly1305: clarify the correct attribution for the AMD64 assembly.

I pushed from the wrong terminal when addressing the comments on
https://golang.org/cl/31256/

Change-Id: Ic75f8348294ed0e4f77a59de948096269a454bcb
Reviewed-on: https://go-review.googlesource.com/31256
Reviewed-by: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

golang/crypto,golang/crypto,ScriptRock/crypto,ScriptRock/crypto,golang/crypto