Removed unused component importing

Importing of TabBarBottom removed because it is not used in the example

Author: asdullahsiddique

docs/tab-based-navigation.md

@@ -136,7 +136,7 @@ class SettingsScreen extends React.Component {
 Usually tabs don't just display one screen — for example, on your Twitter feed, you can tap on a tweet and it brings you to a new screen within that tab with all of the replies. You can think of this as there being separate navigation stacks within each tab, and that's exactly how we will model it in React Navigation.
 
 ```js
-import { TabNavigator, TabBarBottom, StackNavigator } from 'react-navigation';
+import { TabNavigator, StackNavigator } from 'react-navigation';
 
 class DetailsScreen extends React.Component {
   render() {