docs: update install guide

packages/vant/docs/markdown/quickstart.en-US.md

@@ -7,21 +7,21 @@
 Using `npm` to install:
 
 ```bash
-# install Vant 2 for Vue 2 project
-npm i vant@2
+# install latest Vant for Vue 3 project
+npm i vant
 
-# install Vant 3 for Vue 3 project
-npm i vant@3
+# install Vant 2 for Vue 2 project
+npm i vant@latest-v2
 ```
 
 Using `yarn` or `pnpm`:
 
 ```bash
 # with yarn
-yarn add vant@3
+yarn add vant
 
 # with pnpm
-pnpm add vant@3
+pnpm add vant
 ```
 
 ### CDN

packages/vant/docs/markdown/quickstart.zh-CN.md

@@ -11,21 +11,21 @@
 在现有项目中使用 Vant 时，可以通过 `npm` 进行安装：
 
 ```bash
-# Vue 2 项目，安装 Vant 2
-npm i vant@2
+# Vue 3 项目，安装最新版 Vant
+npm i vant
 
-# Vue 3 项目，安装 Vant 3
-npm i vant@3
+# Vue 2 项目，安装 Vant 2
+npm i vant@latest-v2
 ```
 
 当然，你也可以通过 `yarn` 或 `pnpm` 进行安装：
 
 ```bash
 # 通过 yarn 安装
-yarn add vant@3
+yarn add vant
 
 # 通过 pnpm 安装
-pnpm add vant@3
+pnpm add vant
 ```
 
 ### 通过 CDN 安装