fix Firefox bug

Summary:
The motivation is that the getting started page was not working in some cases in Firefox.

This line of code appears to be at best a no-op, at worst fails in Firefox, since "event" is undefined.
Closes https://github.com/facebook/react-native/pull/8335

Differential Revision: D3473333

Pulled By: JoelMarcey

fbshipit-source-id: 40581e83126675aa072c6ee25609cfb787015ce7

docs/QuickStart-GettingStarted.md

@@ -258,7 +258,6 @@ function display(type, value) {
   var container = document.getElementsByTagName('block')[0].parentNode;
   container.className = 'display-' + type + '-' + value + ' ' +
     container.className.replace(RegExp('display-' + type + '-[a-z]+ ?'), '');
-  event && event.preventDefault();
 }
 
 // If we are coming to the page with a hash in it (i.e. from a search, for example), try to get