Removed navigationImage now it's optional

Used to throw without it but now better to leave it empty so shows the default Android search icon

documentation/native/navigation-bar.html

@@ -101,9 +101,7 @@ <h2>Note</h2>
             </div>
             <pre><code class="language-jsx">import {SearchBar} from 'navigation-react-native';
 
-&lt;NavigationBar
-  navigationImage={require('./search.png')}
-  title="Inbox">
+&lt;NavigationBar title="Inbox">
   &lt;SearchBar
     toolbar={true}
     text={text}