chore: use md for frontmatter code fence

Author: meteorlxy

docs/guide/page.md

@@ -30,7 +30,7 @@ Take the `docs` directory as your [sourceDir](../reference/cli.md), e.g. your ar
 
 A markdown file could contain a [YAML](https://yaml.org/) frontmatter. The frontmatter must be at the top of the Markdown file and must be wrapped with a couple of triple-dashed lines. Here is a basic example:
 
-```yaml
+```md
 ---
 lang: en-US
 title: Title of this page

docs/reference/frontmatter.md

@@ -46,14 +46,16 @@
 
 - Example:
 
-```yaml
+```md
+---
 head:
   - - meta
     - name: foo
       content: bar
   - - link
     - rel: canonical
       href: foobar
+---
 ```
 
   Rendered as:
@@ -127,7 +129,7 @@ head:
 
     The page frontmatter is:
 
-```yaml
+```md
 ---
 date: 2021-01-03
 permalinkPattern: :year/:month/:day/:slug.html
@@ -142,7 +144,7 @@ permalinkPattern: :year/:month/:day/:slug.html
 
     The page frontmatter is:
 
-```yaml
+```md
 ---
 permalinkPattern: :year/:month/:day/:slug.html
 ---

docs/zh/guide/page.md

@@ -30,7 +30,7 @@ VuePress 是以 Markdown 为中心的。你项目中的每一个 Markdown 文件
 
 Markdown 文件可以包含一个 [YAML](https://yaml.org/) Frontmatter 。Frontmatter 必须在 Markdown 文件的顶部，并且被包裹在一对三短划线中间。下面是一个基本的示例：
 
-```yaml
+```md
 ---
 lang: zh-CN
 title: 页面的标题

docs/zh/reference/frontmatter.md

@@ -46,14 +46,16 @@
 
 - 示例：
 
-```yaml
+```md
+---
 head:
   - - meta
     - name: foo
       content: bar
   - - link
     - rel: canonical
       href: foobar
+---
 ```
 
   渲染为：
@@ -127,7 +129,7 @@ head:
 
     页面 Frontmatter 是：
 
-```yaml
+```md
 ---
 date: 2021-01-03
 permalinkPattern: :year/:month/:day/:slug.html
@@ -142,7 +144,7 @@ permalinkPattern: :year/:month/:day/:slug.html
 
     页面 Frontmatter 是：
 
-```yaml
+```md
 ---
 permalinkPattern: :year/:month/:day/:slug.html
 ---