docs: fix some outdated links

Author: ulivz

Diff for: packages/docs/docs/plugin/option-api.md

@@ -225,7 +225,7 @@ module.exports = {
 
 **Also see:**
 
-- [Internal plugins in VuePress](https://github.com/vuejs/vuepress/blob/next/packages/%40vuepress/core/lib/markdown/index.js)
+- [Internal markdown-it plugins in VuePress](https://github.com/vuejs/vuepress/blob/master/packages/%40vuepress/markdown/lib/index.js)
 - [Config plugins](https://github.com/neutrinojs/webpack-chain#config-plugins)
 
 ## enhanceAppFiles

Diff for: packages/docs/docs/zh/guide/permalinks.md

@@ -67,4 +67,4 @@ Hello!
 |:regular|VuePress默认的生成永久链接的方式，具体实现看 [这里][2]
 
 [1]:https://en.wikipedia.org/wiki/Link_rot
-[2]:https://github.com/vuejs/vuepress/blob/next/packages/%40vuepress/shared-utils/lib/fileToPath.js
+[2]:https://github.com/vuejs/vuepress/blob/master/packages/%40vuepress/shared-utils/lib/fileToPath.js

Diff for: packages/docs/docs/zh/plugin/option-api.md

@@ -229,7 +229,7 @@ module.exports = {
 
 **参考:**
 
-- [VuePress 的内置 markdown-it 插件](https://github.com/vuejs/vuepress/blob/next/packages/%40vuepress/core/lib/markdown/index.js)
+- [VuePress 的内置 markdown-it 插件](https://github.com/vuejs/vuepress/blob/master/packages/%40vuepress/markdown/lib/index.js)
 - [配置插件](https://github.com/neutrinojs/webpack-chain#config-plugins)
 
 ## enhanceAppFiles