docs: Coroutines guide: update URL

[murfel]

The original URL
https://play.kotlinlang.org/hands-on/Introduction%20to%20Coroutines%20and%20Channels

redirects to
https://kotlinlang.org/docs/kotlin-hands-on.html,

and the Introduction to Kotlin coroutines and channels subsection links you straight back to
https://kotlinlang.org/docs/coroutines-and-channels.html

So I just applied transitive closure :)

Also, this way it becomes consistent with the left-hand-side navigation pane here:

[murfel]

I did run ./gradlew knit as advised, from the repository's root, but it didn't produce any changes.

[dkhalanskyjb]

Thanks!