fix：error in Vuex document link

[heygsc]

Vuex's document, edit this page on GitHub, link to error. This bug comes from the configuration issue with VuePress. The original configured document branch was 4.0, but this branch is no longer visible. This pr changed it to the default branch.

issue : https://github.com/vuejs/vuex/issues/2230

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[weier910]

你的邮件我已收到！

[cuebit]

Good spot. Thanks!

[heygsc]

Good spot. Thanks!

thank you!