Additional link to our flu bragging slide

We currently link to our [flu bragging slide](http://www.cs.cmu.edu/~roni/CDC%20Flu%20Challenge%202014-2018%20Results.pdf) from our [flu page](https://delphi.cmu.edu/flu/).  We should also link to it from our [About page](https://delphi.cmu.edu/about/), specifically from the phrase "perennial leaders in forecasting accuracy".
