chore(docs): change json response field name from project_access to access_level

By trying out system hooks in 10.7.1 we see that the field project_access was probably renamend to access_level.

doc/system_hooks/system_hooks.md

@@ -138,7 +138,7 @@ Please refer to `group_rename` and `user_rename` for that case.
                   "created_at": "2012-07-21T07:30:56Z",
                   "updated_at": "2012-07-21T07:38:22Z",
                   "event_name": "user_add_to_team",
-              "project_access": "Master",
+                "access_level": "Master",
                   "project_id": 74,
                 "project_name": "StoreCloud",
                 "project_path": "storecloud",