docs: modify the example usage of postcssIsolateStyles (#3111)

Co-authored-by: Divyansh Singh <40380293+brc-dd@users.noreply.github.com>

docs/guide/markdown.md

@@ -220,8 +220,6 @@ Wraps in a <div class="vp-raw">
 
 `vp-raw` class can be directly used on elements too. Style isolation is currently opt-in:
 
-::: details
-
 - Install `postcss` with your preferred package manager:
 
   ```sh
@@ -234,9 +232,9 @@ Wraps in a <div class="vp-raw">
   import { postcssIsolateStyles } from 'vitepress'
   
   export default {
-    plugins: {
+    plugins: [
       postcssIsolateStyles()
-    }
+    ]
   }
   ```
 
@@ -248,8 +246,6 @@ Wraps in a <div class="vp-raw">
   })
   ```
 
-:::
-
 ## Syntax Highlighting in Code Blocks
 
 VitePress uses [Shiki](https://shiki.matsu.io/) to highlight language syntax in Markdown code blocks, using coloured text. Shiki supports a wide variety of programming languages. All you need to do is append a valid language alias to the beginning backticks for the code block: