Change links to CodeSandbox in readme to point to 3.x

README.md

@@ -133,8 +133,8 @@ React-admin is designed as a library of loosely coupled React components built o
 
 There are several examples inside the `examples` folder:
 
-* `simple` ([CodeSandbox](https://codesandbox.io/s/github/marmelab/react-admin/tree/master/examples/simple)): a simple application with posts, comments and users that we use for our e2e tests.
-* `tutorial` ([CodeSandbox](https://codesandbox.io/s/github/marmelab/react-admin/tree/master/examples/tutorial)): the application built while following the tutorial.
+* `simple` ([CodeSandbox](https://codesandbox.io/s/github/marmelab/react-admin/tree/3.x/examples/simple)): a simple application with posts, comments and users that we use for our e2e tests.
+* `tutorial` ([CodeSandbox](https://codesandbox.io/s/github/marmelab/react-admin/tree/3.x/examples/tutorial)): the application built while following the tutorial.
 * `demo`: ([Live](https://marmelab.com/react-admin-demo/)) A fictional poster shop admin, serving as the official react-admin demo.
 
 You can run those example applications by calling: