[x/tour] 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 <dmitshur@golang.org>
X-Tour-Commit: 8f38c9a8d074c1943ede6463d78b0d769331dd3e

tour/README.md

@@ -33,9 +33,11 @@ Your browser should now open. If not, please visit [http://localhost:3999/](http
 This repository uses Gerrit for code changes. To learn how to submit changes to
 this repository, see https://golang.org/doc/contribute.html.
 
-The main issue tracker for the tour is located at
-https://github.com/golang/go/issues. Prefix your issue with "tour:" in the
-subject line, so it is easy to find.
+The issue tracker for the tour's code is located at https://github.com/golang/go/issues.
+Prefix your issue with "x/tour:" in the subject line, so it is easy to find.
+
+Issues with the tour's content itself should be reported in the issue tracker 
+at https://github.com/golang/tour/issues.
 
 ## Deploying