Fix the dead link from group usage_quotas#seats-quota-tab

What does this MR do and why?

Clicking on the tooltip for seats on the group usage quota page takes me to a docs page that returns a 404:

https://docs.gitlab.com/ee/subscription/gitlab_com/index.html#how-seat-usage-is-determined

That link should be https://docs.gitlab.com/ee/subscriptions/gitlab_com/index.html#how-seat-usage-is-determined .

Refs.

Updates !82399 (merged) (and !85788 (merged))

Source: https://gitlab.slack.com/archives/CETG54GQ0/p1654525956181779