docs: fix site link ref to v4 github link

docs/react/customize-theme.en-US.md

@@ -11,7 +11,7 @@ Ant Design allows you to customize our design tokens to satisfy UI diversity fro
 
 We are using [Less](http://lesscss.org/) as the development language for styling. A set of less variables are defined for each design aspect that can be customized to your needs.
 
-There are some major variables below, all less variables could be found in [Default Variables](https://github.com/ant-design/ant-design/blob/master/components/style/themes/default.less).
+There are some major variables below, all less variables could be found in [Default Variables](https://github.com/ant-design/ant-design/blob/4.x-stable/components/style/themes/default.less).
 
 ```less
 @primary-color: #1890ff; // primary color for all components

docs/react/customize-theme.zh-CN.md

@@ -11,7 +11,7 @@ Ant Design 设计规范和技术上支持灵活的样式定制，以满足业务
 
 antd 的样式使用了 [Less](http://lesscss.org/) 作为开发语言，并定义了一系列全局/组件的样式变量，你可以根据需求进行相应调整。
 
-以下是一些最常用的通用变量，所有样式变量可以在 [这里](https://github.com/ant-design/ant-design/blob/master/components/style/themes/default.less) 找到。
+以下是一些最常用的通用变量，所有样式变量可以在 [这里](https://github.com/ant-design/ant-design/blob/4.x-stable/components/style/themes/default.less) 找到。
 
 ```less
 @primary-color: #1890ff; // 全局主色