🦊 Update docs to reflect correct payload types.

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA

What does this MR do and why?

🦊 Update docs to reflect correct payload types.

Hello, I recently began working on some webhooks and noticed and issue.

• When I open an MR I get opened
• When I reopen a MR I get opened
• When I update a MR I get opened
• When I close a MR I get closed
• When I merge a MR I get merged

How to set up and validate locally

Docs page