v4 Docs: ReadMe.md link to React Router incorrect

[nickcoleman]

Ref Section: In your How do I access router state in a container component?, 1st sentence

"React Router provides route information via a route component's props."

The link to the React Router docs is incorrect.
Current: https://github.com/ReactTraining/react-router/blob/master/docs/Introduction.md#getting-url-parameters
Should be: https://github.com/ReactTraining/react-router/blob/v3/docs/Introduction.md#getting-url-parameters