go.tools/cmd/vet: fix error message
go.tools/cmd/vet: fix error message

R=r, dsymonds, bradfitz
CC=golang-dev
https://codereview.appspot.com/11552043

tmc/mirror-go.tools,t-yuki/go.tools.old,t-yuki/go.tools.old,tmc/mirror-go.tools,t-yuki/go.tools.old,tmc/mirror-go.tools