docs(babel): update export in README (#690)

Author: Maopy

packages/babel/README.md

@@ -39,7 +39,7 @@ npm install @rollup/plugin-babel --save-dev
 Create a `rollup.config.js` [configuration file](https://www.rollupjs.org/guide/en/#configuration-files) and import the plugin:
 
 ```js
-import babel from '@rollup/plugin-babel';
+import { babel } from '@rollup/plugin-babel';
 
 const config = {
   input: 'src/index.js',