Fix unresponsive reply button in discussions

What does this MR do?

This fixes a regression introduced when discussion notes were refactored in this MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27066 Due to incorrect casing used in events handlers, the reply button became unresponsive:

The casing has now been corrected, which restores the button's functionality:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Style guides

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes #63200 (closed)