Fix example for withRouter

No Login component has been defined, i guess "Page" was meant

Author: anonymosh

upgrade-guides/v2.4.0.md

@@ -25,7 +25,7 @@ const Page = React.createClass({
 
 })
 
-export default withRouter(Login)
+export default withRouter(Page)
 ```
 
 **It's important to note this is not a deprecation of the `context` API.** As long as React supports `this.context` in its current form, any code written for that API will continue to work. We will continue to use it internally and you can continue to write in that format, if you want. We think this new HoC is nicer and easier, and will be using it in documentation and examples, but it is not a hard requirement to switch.