go-tour: update sub-repo import path I missed the appengine one in last CL.
go-tour: update sub-repo import path
I missed the appengine one in last CL.

LGTM=adg
R=adg
CC=golang-codereviews
https://codereview.appspot.com/176670043

minux/go-tour-zh,minux/go-tour-zh,Go-zh/tour-old,Go-zh/tour-old