Minor change on code style of readme (#304)

Use one type of quotes in the README file.

Author: ImanMh

README.md

@@ -36,7 +36,7 @@ The development UMD build also needs the package `prop-types` being loaded besid
 
 ```js
 import { Tab, Tabs, TabList, TabPanel } from 'react-tabs';
-import "react-tabs/style/react-tabs.css";
+import 'react-tabs/style/react-tabs.css';
 
 export default () => (
   <Tabs>