Fix broken link to default layout in Theming documentation

docs/Theming.md

@@ -456,7 +456,7 @@ const MyLayout = props => <Layout
 
 ### Layout From Scratch
 
-For more custom layouts, write a component from scratch. It must contain a `{children}` placeholder, where react-admin will render the resources. Use the [default layout](https://github.com/marmelab/react-admin/blob/master/src/mui/layout/Layout.js) as a starting point. Here is a simplified version (with no responsive support):
+For more custom layouts, write a component from scratch. It must contain a `{children}` placeholder, where react-admin will render the resources. Use the [default layout](https://github.com/marmelab/react-admin/blob/master/packages/ra-ui-materialui/src/layout/Layout.js) as a starting point. Here is a simplified version (with no responsive support):
 
 ```jsx
 // in src/MyLayout.js