Add 'converted_to_draft' action to the pull request webhook payload documentation. (#2417)

Co-authored-by: Sarah Edwards <skedwards88@github.com>

Author: recurser

data/reusables/webhooks/pull_request_webhook_properties.md

@@ -1,3 +1,3 @@
 Key | Type | Description
 ----|------|-------------
-`action`|`string` | The action that was performed. Can be one of  `opened`, `edited`, `closed`, `assigned`, `unassigned`, `review_requested`, `review_request_removed`, `ready_for_review`, `labeled`, `unlabeled`, `synchronize`, `locked`, `unlocked`, or `reopened`. If the action is `closed` and the `merged` key is `false`, the pull request was closed with unmerged commits. If the action is `closed` and the `merged` key is `true`, the pull request was merged.
+`action`|`string` | The action that was performed. Can be one of  `opened`, `edited`, `closed`, `assigned`, `unassigned`, `review_requested`, `review_request_removed`, `ready_for_review`, `converted_to_draft`, `labeled`, `unlabeled`, `synchronize`, `locked`, `unlocked`, or `reopened`. If the action is `closed` and the `merged` key is `false`, the pull request was closed with unmerged commits. If the action is `closed` and the `merged` key is `true`, the pull request was merged.