update export example

In order to provide perfect compatibility with `require` the export needs to be done in both styles:

```js
let BuildClassifier = {};

BuildClassifier.answer = 42;

export let answer = BuildClassifier.answer;
export default BuildClassifier;
```

查找 issue #35 引用的导出示例，并检查该包目前如何公开其值。更新示例，以展示适用于 require 兼容性的命名导出和默认导出两种风格，然后验证文档中的示例是否符合预期的 JavaScript 用法。

由索引模型根据 Issue 内容生成。