Update build triggers API documentation (#10153)

Update build triggers API documentation
[ci skip]

* Ensure updated/deleted timestamps are consistent

doc/api/build_triggers.md

@@ -101,8 +101,18 @@ DELETE /projects/:id/triggers/:token
 | Attribute | Type    | required | Description              |
 |-----------|---------|----------|--------------------------|
 | `id`      | integer | yes      | The ID of a project      |
-| `token`   | string  | yes      | The `token` of a project |
+| `token`   | string  | yes      | The `token` of a trigger |
 
 ```
 curl -X DELETE -H "PRIVATE_TOKEN: 9koXpg98eAheJpvBs5tK" "https://gitlab.example.com/api/v3/projects/1/triggers/7b9148c158980bbd9bcea92c17522d"
 ```
+
+```json
+{
+    "created_at": "2015-12-23T16:25:56.760Z",
+    "deleted_at": "2015-12-24T12:32:20.100Z",
+    "last_used": null,
+    "token": "7b9148c158980bbd9bcea92c17522d",
+    "updated_at": "2015-12-24T12:32:20.100Z"
+}
+```