go.tools/go/types: fix typo in comment
go.tools/go/types: fix typo in comment

TBR=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/135960043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools