Distribution docs: fix mention of `alpha`

This item is at the top of doc/advanced/multiple-databases/index.md. It doesn't match our current pattern of experiment / beta / GA. A TW with knowledge of the feature's current status (experiment? beta?) should update it while fixing the link.

WARNING:
This feature is not ready for production use. Due to 
[known issues](https://docs.gitlab.com/ee/administration/postgresql/multiple_databases.html#known-issues), 
configuring GitLab with multiple databases is in 
[**Alpha**](https://docs.gitlab.com/ee/policy/alpha-beta-support.html#alpha-features).

Further details

Proposal

Who can address the issue

Other links/references

• Related to gitlab-org/technical-writing#1006
• Related to Round 9: fix links (and table) in Charts repo (!3654 - merged) - and specifically !3654 (comment 1802240736) - where the need was spotted