docs: update link to fileToPath.js (#995)

Actual link to :regular implementation is broken

Author: massimilianomarini

packages/docs/docs/guide/permalinks.md

@@ -71,4 +71,4 @@ Hello!
 |:day|Published day of posts (2-digit)|
 |:i_day|Published day of posts (Without leading zeros)|
 |:slug|Slugified file path (Without extension)|
-|:regular| Permalink generated by VuePress by default, for implementation see [here](https://github.com/vuejs/vuepress/blob/next/packages/%40vuepress/shared-utils/lib/fileToPath.js) |
+|:regular| Permalink generated by VuePress by default, for implementation see [here](https://github.com/vuejs/vuepress/blob/master/packages/%40vuepress/shared-utils/lib/fileToPath.js) |