Update teams feature availability for GA/self-service. (#3908)

Author: chadwcarlson

themes/psh-docs/layouts/partials/user-mgt-sellable/body.md

@@ -1,4 +1,4 @@
 {{ $content := `This feature is available as part of the Standard User Management add-on.
-To add it to your project, [contact Sales](https://platform.sh/contact/).`}}
+To add it to your project, [administer your organization's billing](/administration/billing/billing-admin).`}}
 
 {{ partial "premium-features/banner" ( dict "context" . "content" $content "title" "Feature Availability" )}}