Update Auto-Devops docs to mention new REPLICAS env var

What does this MR do?

This part is strictly about correcting a previous mistake in the docs.

The implementation is done in gitlab-ci-yml!151 (merged)

And the docs for this feature will need to live in EE codebase (since they don't make much sense in CE right now). The MR for EE docs is https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5143

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [-] API support added
• [-] Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [-] Internationalization required/considered
• [-] End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#45228 (closed)