Fix typo in issue_board.md

What does this MR do?

Fix typo in gitlab help (issue_board.md). No changes to code whatsoever, so I assume I can leave the checkboxes below unticked (?).

Screenshots

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

Availability and Testing

Security