Doc: fix custom theme, fixed #1828 (#1837)

examples/docs/en-US/custom-theme.md

@@ -20,7 +20,7 @@ npm i https://github.com/ElementUI/theme-default -D
 After successfully installing the above packages, a command named `et` is available in CLI (if the packages are installed locally, use `node_modules/.bin/et` instead). Run `-i` to initialize the variable file which outputs to `element-variables.css` by default. And you can specify its output directory as you will.
 
 ```shell
-et -i [custom output directory]
+et -i [custom output file]
 
 > ✔ Generator variables file
 ```

examples/docs/zh-CN/custom-theme.md

@@ -20,7 +20,7 @@ npm i https://github.com/ElementUI/theme-default -D
 主题生成工具安装成功后，如果全局安装可以在命令行里通过 `et` 调用工具，如果安装在当前目录下，需要通过 `node_modules/.bin/et` 访问到命令。执行 `-i` 初始化变量文件。默认输出到 `element-variables.css`，当然你可以传参数指定文件输出目录。
 
 ```shell
-et -i [可以自定义变量文件目录]
+et -i [可以自定义变量文件]
 
 > ✔ Generator variables file
 ```