go.crypto/salsa: fix typo in comment and move TODO out of package comment.
go.crypto/salsa: fix typo in comment and move TODO out of package comment.

Thanks to Frithjof Schulze for noticing.

R=goland-dev, r
CC=golang-dev
https://codereview.appspot.com/11033044

polydawn-ports/go.crypto,tonnerre/golang-go.crypto,dizzyd/go.crypto,drhax9908/go.crypto,xman1101/terminal,cmars/go.crypto,xman1101/terminal,drhax9908/go.crypto,polydawn-ports/go.crypto,tonnerre/golang-go.crypto,cmars/go.crypto,isislovecruft/go.crypto,isislovecruft/go.crypto,dizzyd/go.crypto