docs: fix href to v3 documentation

misdirect me once, blame on me. misdirect me twice, submit a PR.

Author: CRdeGraaf

Diff for: README.md

@@ -71,7 +71,7 @@ Vue Router is part of the Vue Ecosystem and is an MIT-licensed open source proje
 
 ---
 
-Get started with the [documentation](http://router.vuejs.org), or play with the [examples](https://github.com/vuejs/vue-router/tree/dev/examples) (see how to run them below).
+Get started with the [documentation](http://v3.router.vuejs.org), or play with the [examples](https://github.com/vuejs/vue-router/tree/dev/examples) (see how to run them below).
 
 ### Development Setup