close iframe tag

If you don't do so, nbviewer fails to show any document beyond the iframe tag. 

[This talk](http://nbviewer.ipython.org/github/llimllib/fantasypl/blob/7a1f9ad6ddbc917bd27813c7f4cbd45afcd156e5/talk/Fantasy%20Football%20Presentation.ipynb) demonstrates the problem. Scroll down to the bottom, and you'll see that the talk appears to end after the iframe.

After the fix, it looks like [this](http://nbviewer.ipython.org/github/llimllib/fantasypl/blob/master/talk/Fantasy%20Football%20Presentation.ipynb); you'll see it continues past the iframe.