fix(docz-plugin-babel6): documentation for babel plugin (#60)

* rename plugin export to babel6

* revert plugin and update docs

packages/docz-plugin-babel6/README.md

@@ -6,6 +6,6 @@ By default and by some performance issues, Docz use Babel@7. If you want to use
 import { babel } from 'docz-plugin-babel6'
 
 export default {
-  plugins: [babel6()]
+  plugins: [babel()]
 }
 ```