Fix capitalization of Dropbox (donnemartin#511)

README.md

@@ -1723,7 +1723,7 @@ Handy metrics based on numbers above:
 | Amazon | [Amazon architecture](http://highscalability.com/amazon-architecture) |
 | Cinchcast | [Producing 1,500 hours of audio every day](http://highscalability.com/blog/2012/7/16/cinchcast-architecture-producing-1500-hours-of-audio-every-d.html) |
 | DataSift | [Realtime datamining At 120,000 tweets per second](http://highscalability.com/blog/2011/11/29/datasift-architecture-realtime-datamining-at-120000-tweets-p.html) |
-| DropBox | [How we've scaled Dropbox](https://www.youtube.com/watch?v=PE4gwstWhmc) |
+| Dropbox | [How we've scaled Dropbox](https://www.youtube.com/watch?v=PE4gwstWhmc) |
 | ESPN | [Operating At 100,000 duh nuh nuhs per second](http://highscalability.com/blog/2013/11/4/espns-architecture-at-scale-operating-at-100000-duh-nuh-nuhs.html) |
 | Google | [Google architecture](http://highscalability.com/google-architecture) |
 | Instagram | [14 million users, terabytes of photos](http://highscalability.com/blog/2011/12/6/instagram-architecture-14-million-users-terabytes-of-photos.html)<br/>[What powers Instagram](http://instagram-engineering.tumblr.com/post/13649370142/what-powers-instagram-hundreds-of-instances) |