Fix phpdoc for DataList::relation method (namespace)

[JorisDebonnet]

Fixed an old non-namespaced phpdoc.

The example from that doc still doesn't seem to work though. Perhaps I should create an issue about that, but you never know someone here knows what's wrong.

When I try DataList::Create(\SilverStripe\Security\Group::class)->relation("Members")->Count() it returns 0. Even on a brand-new installation, it should at least return the Default Administrator (belonging to the Administrator group), shouldn't it? At least, \SilverStripe\Security\Group::get()->byID(2)->Members()->Count() does return 1.

(I'm trying to fix an issue in symbiote/silverstripe-memberprofiles where I have the same issue of relation("Members") returning an empty list while expecting Members)

[dhensby]

This docs fix looks good, so I'll merge it - I'm really not sure about the bug you describe, a new issue for that would be good.