Update BrowserHistory.md

`new BrowserHistory()` shouldn't be inside the props decleration but outside the render method.

Author: Schniz

doc/03 History/BrowserHistory.md

@@ -44,9 +44,10 @@ Example
 ```js
 import { Router } from 'react-router';
 import BrowserHistory from 'react-router/lib/BrowserHistory';
+var history = new BrowserHistory();
 
 React.render((
-  <Router history={new BrowserHistory}>
+  <Router history={ history }>
     {/* ... */}
   </Router>
 ), document.body);