updates default preset link (#1668)

the previous link currently gives a 404

Author: edm00se

packages/docs/docs/config/README.md

@@ -135,7 +135,7 @@ module.exports = {
 
 ### palette.styl
 
-If you wish to apply simple color overrides to the styling of the [default preset](https://github.com/vuejs/vuepress/blob/master/packages/@vuepress/core/lib/app/style/config.styl) or define some color variables for using later, you can create an `.vuepress/styles/palette.styl` file.
+If you wish to apply simple color overrides to the styling of the [default preset](https://github.com/vuejs/vuepress/blob/master/packages/@vuepress/core/lib/client/style/config.styl) or define some color variables for using later, you can create an `.vuepress/styles/palette.styl` file.
 
 There are a few color variables you can tweak: