fix small mistake

https://github.com/reactjs/react-tabs/blob/master/src/helpers/elementTypes.js#L2

Author: David Narbutovich

README.md

@@ -346,7 +346,7 @@ const CustomTab = ({ children }) => (
   </Tab>
 );
 
-CustomTab.tabsRosle = 'Tab'; // Required field to use your custom Tab
+CustomTab.tabsRole = 'Tab'; // Required field to use your custom Tab
 
 const App = () => (
   <Tabs>