Update Bug_report.md to llink to v4 and v5 sandboxes

Author: fzaninotto

.github/ISSUE_TEMPLATE/Bug_report.md

@@ -22,8 +22,8 @@ about: Something isn't working as expected. Please tell us!
 <!-- If you are able to illustrate the bug or feature request with an example, please provide a sample application via one of the following means: -->
 
 * Preferably, a sandbox forked from 
-  - https://stackblitz.com/github/marmelab/react-admin/tree/master/examples/simple (v4)
-  - https://codesandbox.io/s/github/marmelab/react-admin/tree/3.x/examples/simple (v3)
+  - https://stackblitz.com/github/marmelab/react-admin/tree/master/examples/simple (v5)
+  - https://stackblitz.com/github/marmelab/react-admin/tree/4.x/examples/simple (v4)
 * A link to a GitHub repo with the minimal codebase to reproduce the issue