runtime: fix minor doc typos in amd64 memmove
runtime: fix minor doc typos in amd64 memmove

Change-Id: Ic1ce2f93d6a225699e9ce5307d62cdda8f97630d
Reviewed-on: https://go-review.googlesource.com/94024
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cloudflare/go,CAFxX/go,nawawi/go,codestation/go,nawawi/go,josharian/go,josharian/go,codestation/go,cloudflare/go,nawawi/go,golang/go,josharian/go,akutz/go,Cofyc/go,cloudflare/go,codestation/go,cloudflare/go,Cofyc/go,cloudflare/go,codestation/go,CAFxX/go,nawawi/go,josharian/go,golang/go,Cofyc/go,CAFxX/go,nawawi/go,codestation/go,Cofyc/go,akutz/go,akutz/go,josharian/go,CAFxX/go,akutz/go,cloudflare/go,Cofyc/go,golang/go,akutz/go,Cofyc/go,nawawi/go,CAFxX/go,josharian/go,cloudflare/go,cloudflare/go,josharian/go,akutz/go,nawawi/go,codestation/go,CAFxX/go,akutz/go,golang/go,josharian/go,codestation/go,codestation/go,golang/go,golang/go,akutz/go,golang/go,Cofyc/go,CAFxX/go,golang/go,Cofyc/go,CAFxX/go,nawawi/go