fix reference bug
Merge branch 'master' of https://github.com/jessicatysu/cs224w
Merge branch 'master' of https://github.com/jessicatysu/cs224w
Merge branch 'master' of https://github.com/jessicatysu/cs224w
Merge branch 'master' of https://github.com/jessicatysu/cs224w