update adding-a-router.md for react-router-dom v6

Created by: lookto

Updated the hyperlinks in the documentation to refer to the react-router-dom example and tutorial for v6.

docusaurus/docs/adding-a-router.md

@@ -17,6 +17,6 @@ Alternatively you may use `yarn`:
 yarn add react-router-dom
 ```
 
-To try it, delete all the code in `src/App.js` and replace it with any of the examples on its website. The [Basic Example](https://reacttraining.com/react-router/web/example/basic) is a good place to get started.
+To try it, delete all the code in `src/App.js` and replace it with any of the examples on its website. The [Basic Example](https://reactrouter.com/docs/en/v6/examples/basic) or the [Tutorial](https://reactrouter.com/docs/en/v6/getting-started/tutorial) are good places to get started. 
 
 Note that [you may need to configure your production server to support client-side routing](deployment.md#serving-apps-with-client-side-routing) before deploying your app.