[x/tour] content: link to working and updated korean translation

The korean translation we link to is offline due to the

  Go 1.9 is no longer available.

error; but even before that it was quite outdated (last re-deployment
was ~6 years ago).

A few people have been working on an updated translation, which they
deployed at go-tour-ko.appspot.com (repo github.com/golang-ko/tour).

I don't speak korean but I've skimmed through it and it looks fine.
Link to the new one.

Updates golang/tour#1026
Fixes golang/tour#709
Fixes golang/tour#1027

Change-Id: I08edb12737a82381aeb33bb3f324e87b92936115
Reviewed-on: https://go-review.googlesource.com/c/tour/+/253337
Reviewed-by: Alberto Donizetti <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Trust: Alberto Donizetti <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Go Bot <[email protected]>
X-Tour-Commit: 6f9d4ff994b136951d420708cd231da267653086

tour/content/welcome.article

@@ -59,7 +59,7 @@ The tour is available in other languages:
 - [[https://go-tour-id2.appspot.com/][Indonesian — Bahasa Indonesia]]
 - [[https://go-tour-ita.appspot.com/][Italian — Italiano]]
 - [[https://go-tour-jp.appspot.com/][Japanese — 日本語]]
-- [[https://go-tour-kr.appspot.com/][Korean — 한국어]]
+- [[https://go-tour-ko.appspot.com/][Korean — 한국어]]
 - [[https://go-tour-ro.appspot.com/][Romanian — Română]]
 - [[https://go-tour-ru-ru.appspot.com/][Russian - Русский]]
 - [[https://gotour-es.appspot.com/][Spanish — Español]]