Add information about The Art Of PostgreSQL training

What does this MR do?

This MR adds information about The Art of PostgreSQL being available for GitLab team members

doc/development/database/database_reviewer_guidelines.md

@@ -62,7 +62,7 @@ The following guides provide a quick introduction and links to follow on more ad
 - Guide on [understanding EXPLAIN plans](../understanding_explain_plans.md).
 - [Explaining the unexplainable series in `depesz`](https://www.depesz.com/tag/unexplainable/).
 
-We also have unlimited access to The Art of PostgreSQL available, if you are interested in getting access please check out the
+We also have licensed access to The Art of PostgreSQL available, if you are interested in getting access please check out the
 [issue (confidential)](https://gitlab.com/gitlab-org/database-team/team-tasks/-/issues/23).
 
 Finally, you can find various guides in the [Database guides](index.md) page that cover more specific