chore: update readme links [ci skip]

Author: yyx990803

README.md

@@ -1,11 +1,11 @@
 # Vuex
 
 [![npm](https://img.shields.io/npm/v/vuex.svg)](https://npmjs.com/package/vuex)
-[![ci status](https://circleci.com/gh/vuejs/vuex/tree/dev.png?style=shield)](https://circleci.com/gh/vuejs/vuex)
+[![ci status](https://circleci.com/gh/vuejs/vuex/tree/3.x.png?style=shield)](https://circleci.com/gh/vuejs/vuex)
 
 ---
 
-:fire: **HEADS UP!** You're currently looking at Vuex 3 branch. If you're looking for Vuex 4, [please check out `4.0` branch](https://github.com/vuejs/vuex/tree/4.0).
+:fire: **HEADS UP!** You're currently looking at Vuex 3 branch. If you're looking for Vuex 4, [please check out `main` branch](https://github.com/vuejs/vuex/tree/main).
 
 ---