node-repo: switch option name
node-repo: switch option name

In an attempt to fix the following error on the production server:

```
11:40:52.491 DEBUG bot: Resolved labels: doc,vm (req_id=47558d90-8c33-11e8-8f2d-6778d538d753, pr=nodejs/node/#21911, action=pull_request.opened)
11:40:52.492 DEBUG bot: Trying to add labels: doc,vm (req_id=47558d90-8c33-11e8-8f2d-6778d538d753, pr=nodejs/node/#21911, action=pull_request.opened)
11:40:52.492 ERROR bot: Error while adding labels (req_id=47558d90-8c33-11e8-8f2d-6778d538d753, pr=nodejs/node/#21911, action=pull_request.opened)
    err: {
      "code": "400",
      "status": "Bad Request",
      "message": "Empty value for parameter 'labels': undefined"
    }
```

PR-URL: https://github.com/nodejs/github-bot/pull/194
Refs: https://github.com/nodejs/github-bot/issues/192
Reviewed-By: Rich Trott <28a7b6144c43947e3f532c033ff42c8a2ae08045@gmail.com>
phillipj/github-bot,nodejs/github-bot