Update migration url

Author: asvarcas

UPGRADE.md

@@ -35,7 +35,7 @@ Failing to upgrade one of the `ra-` packages will result in a duplication of the
 * `react` and `react-dom` are now required to be >= 16.9. This version is backward compatible with 16.3, which was the minimum requirement in react-admin, and it offers the support for Hooks, on which react-admin v3 relies heavily.
 * `react-redux` requires a minimum version of 7.1.0 (instead of 5.0). Check their upgrade guide for [6.0](https://github.com/reduxjs/react-redux/releases/tag/v6.0.0) and [7.0](https://github.com/reduxjs/react-redux/releases/tag/v7.0.0)
 * `redux-saga` requires a minimum version of 1.0.0 (instead of ~0.16.0). Check their [list of breaking changes for redux-saga 1.0](https://github.com/redux-saga/redux-saga/releases/tag/v1.0.0) on GitHub. 
-* `material-ui` requires a minimum of 4.0.0 (instead of 1.5). Check their [Upgrade guide](https://next.material-ui.com/guides/migration-v3/).
+* `material-ui` requires a minimum of 4.0.0 (instead of 1.5). Check their [Upgrade guide](https://mui.com/guides/migration-v3/).
 
 ## `react-router-redux` replaced by `connected-react-router`