Update README.md (#37)

Conflict between unused imported "Item" and const "Item" defined in MyHeaderButtons.js. Just deleted the import to avoid Duplicate Declaration error when building project.

Author: delgadofarid

README.md

@@ -84,7 +84,7 @@ This sections covers how you should use the library in your project. Please note
 
 import * as React from 'react';
 import MaterialIcons from 'react-native-vector-icons/MaterialIcons';
-import HeaderButtons, { HeaderButton, Item } from 'react-navigation-header-buttons';
+import { HeaderButtons, HeaderButton } from 'react-navigation-header-buttons';
 
 // define IconComponent, color, sizes and OverflowIcon in one place
 const MaterialHeaderButton = props => (