docs: install with pnpm (#9850)

packages/vant-markdown-loader/README.md

@@ -4,27 +4,26 @@ Simple and fast vue markdown loader, transform markdown to vue component.
 
 ## Install
 
-### NPM
-
 ```shell
-npm i @vant/markdown-loader -S
-```
+# with npm
+npm i @vant/markdown-loader -D
 
-### YARN
+# with yarn
+yarn add @vant/markdown-loader -D
 
-```shell
-yarn add @vant/markdown-loader
+# with pnpm
+pnpm add @vant/markdown-loader -D
 ```
 
 ## Options
 
 - `enableMetaData`: Default `false`. Whether to use [front-matter](https://github.com/jxson/front-matter) to extract markdown meta data
 
-- `linkOpen`: Default `true`. Whether to add target="_blank" to all links
+- `linkOpen`: Default `true`. Whether to add target="\_blank" to all links
 
 - `wrapper(html, fm)`: Format the returned content using a custom function
-    - `html`: The result of [markdown-it](https://github.com/markdown-it/markdown-it)'s render
-    - `fm`: See [fm(string)](https://github.com/jxson/front-matter#fmstring). If `enableMetaData` option is `false`, the value is `undefined`. 
-        - `attributes`
-        - `body`
-        - `frontmatter`
+  - `html`: The result of [markdown-it](https://github.com/markdown-it/markdown-it)'s render
+  - `fm`: See [fm(string)](https://github.com/jxson/front-matter#fmstring). If `enableMetaData` option is `false`, the value is `undefined`.
+    - `attributes`
+    - `body`
+    - `frontmatter`