Fix undefined method error

What does this MR do?

Fixes an undefined method error in help.rb resulting in a 500 upon a custom slash command from Mattermost.

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

Closes #221052 (closed)