Documentation: update duo chat model usage

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183611



Merged-by: Jon Glassman <jglassman@gitlab.com>
Approved-by: Logan Stucker <lstucker@gitlab.com>
Approved-by: Bruno Cardoso <bcardoso@gitlab.com>
Approved-by: Jon Glassman <jglassman@gitlab.com>
Co-authored-by: Nathan Weinshenker <nweinshenker@gitlab.com>