Link points to old Swift 6 configuration document

[mattmassicotte]

Please describe the bug

The last link at the end of this section points to a page that will eventually be retired.

https://swiftpackageindex.com/docs/builds#data-race-safety

old: https://www.swift.org/documentation/concurrency/ new: https://www.swift.org/migration/documentation/swift-6-concurrency-migration-guide/completechecking

I'm pretty sure it will end up just being a redirect. But, still seems like its worth updating.

Explain the steps needed to reproduce the bug

What was the expected behaviour?

Screenshots

What web browser were you using? (if relevant)

Additional context

[daveverwer]

Thanks @mattmassicotte. Fixed in #3194!