docs: update markdown.extendMarkdown

Author: ulivz

packages/docs/docs/config/README.md

@@ -231,7 +231,7 @@ module.exports = {
 ```
 
 ::: tip
-This option is also included in [Plugin API](../plugin/README.md#extendmarkdown).
+This option is also included in [Plugin API](../plugin/option-api.md#extendmarkdown).
 :::
 
 ## Build Pipeline

packages/docs/docs/zh/config/README.md

@@ -201,7 +201,7 @@ VuePress provides a convenient way to add extra styles. you can create an `.vuep
 
 [markdown-it-table-of-contents](https://github.com/Oktavilla/markdown-it-table-of-contents) 的选项。
 
-### markdown.config
+### markdown.extendMarkdown
 
 - 类型: `Function`
 - 默认值: `undefined`
@@ -219,6 +219,10 @@ module.exports = {
 }
 ```
 
+::: tip
+这个选项也被 [Plugin API](../plugin/option-api.md#extendmarkdown) 所支持。
+:::
+
 ## 构建流程
 
 ### postcss