[ML] Machine Learning Detector help links 404 in 6.8

[loekvangool]

Kibana version: 6.8

Elasticsearch version: 6.8

Describe the bug:
While creating a job, ML gives deeplinks to pages in the manual. However, they 404:

https://www.elastic.co/guide/en/x-pack/6.8/ml-rare-functions.html#ml-freq-rare

I believe it should be:
https://www.elastic.co/guide/en/elastic-stack-overview/6.8/ml-rare-functions.html

Could not reproduce on 7.2, seems to be limited to 6.x.

[elasticmachine]

Pinging @elastic/ml-ui

[peteharverson]

This link was corrected for v7.1.0 onwards in #35734. I will backport that to the 6.8 branch, so it is in for 6.8.2 when that is released.

[peteharverson]

Backported for 6.8.2 in #39426