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 <agl@golang.org>

chacha20poly1305/chacha20poly1305_amd64.s

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// This assembly file was lovingly hand-crafted by Vlad Krasnov of CloudFlare.
+// This file was originally from https://golang.org/cl/24717 by Vlad Krasnov of CloudFlare.
 
 // +build go1.7