Fix typos in comments of relative_positioning.rb

What does this MR do?

Fix typos in comments in app/models/concerns/relative_positioning.rb, introduced in gitlab-foss!21651 (merged).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content