Update event-brokers.mdx

[ZhengHe-MD]

Fix typo

Proposed changes: Just fix a trivial typo, if I'm not mistaken

Status (please check what you already did):

• [ ] added some tests for the functionality
• [x] updated the documentation
• [ ] updated the changelog (please check changelog for instructions)
• [ ] reformat files using black (please check Readme for instructions)

[CLAassistant]

All committers have signed the CLA.

[sara-tagger]

Thanks for submitting a pull request 🚀 @mvielkind will take a look at it as soon as possible ✨

[stale[bot]]

This PR has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.