Update backup/manager.rb to clarify the backup restore string format

What does this MR do?

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

It's just a comment change, so no perf impact.

Security

No security impact.