adding subgroups to docs

Hi @eread, we are adding a fix to contribution analytics, since before it only included groups and not the underlying data from subgroups. I made a very small change to the docs, is that ok?

@johnjeremiah - do you think this is something we should include in 12.2 release post or should only be present in the docs? I would say just docs?

Link to issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/12595 Link to MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14536