fix broken urls: from reacttraining.com to v5.reactrouter.com

[iampreethamveigas]

The PR incudes fix of

1. Access the page with the typo or broken link: https://reacttraining.com/react-router/web/api/Router
   
   
   
2. Verify that the link is not working or that there is a typo.
   
3. Made the change to the link by replacing "https://reacttraining.com/react-router/web/api/Router" with "https://v5.reactrouter.com/web/api/Router)"
   
4. 
5. Verify that the link now works or that the typo has been corrected.