Update Events API PullRequestEvent properties (#58684)

Co-authored-by: Copilot <[email protected]>

Author: jessmargz

data/reusables/webhooks/pull_request_event_api_properties.md

@@ -1,7 +1,7 @@
 Key | Type | Description
 ----|------|-------------
 | {% ifversion fpt or ghec %} |
-`action`|`string` | The action that was performed. Can be one of `opened`, `closed`, `reopened`, `assigned`, `unassigned`, `labeled`, and `unlabeled`.
+`action`|`string` | The action that was performed. Can be one of `opened`, `closed`, `merged`, `reopened`, `assigned`, `unassigned`, `labeled`, or `unlabeled`.
 | {% else %} |
 `action`|`string` | The action that was performed. Can be one of `opened`, `closed`, `reopened`.
 | {% endif %} |