README: clarify where tour issues should be reported
README: clarify where tour issues should be reported

Issues with the tour code should be reported in the tracker at

  github.com/golang/go/issues

While the 'report bug' button in the tour itself points to the
issue tracker at

  github.com/golang/tour/issues

which is the one where issues about the tour's content should
be reported. Make this clear in the README.

Fixes golang/go#36843

Change-Id: I7844ea1a15470fdfbc970a9485d37250a6c38779
Reviewed-on: https://go-review.googlesource.com/c/tour/+/217357
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

zorion/go-tour-ca,dupoxy/go-tour-fr,zorion/go-tour-ca,dupoxy/go-tour-fr,golang/tour,dupoxy/go-tour-fr