[x/blog] blog: update URLs for "A Tour of Go"
[x/blog] blog: update URLs for "A Tour of Go"

The URLs to the Go tour were pointing to http://go-tour.appspot.com/,
this changes them to https://tour.golang.org/

Change-Id: Ied9e0440522cdefdccf5a1daf4563add9a62b8dd
Reviewed-on: https://go-review.googlesource.com/c/blog/+/169937
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>
X-Blog-Commit: da069a0697710daf6b99c8cd4a6d02098d6f86d6

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website