IOFileSystem - problem using the Customize example in the docs for 2.0.0

I'm upgrading from the previous version to 2.0.0 and trying to follow the new Customize instructions to use CacheManager instead of BaseCacheManager. The reference to IOFileSystem doesn't resolve. What am I doing wrong?

```dart
CacheManager(
Config(
key,
stalePeriod: const Duration(days: 7),
maxNrOfCacheObjects: 60,
repo: JsonCacheInfoRepository(databaseName: key),
fileSystem: IOFileSystem(key),
fileService: HttpFileService(),
),
);
```

从 2.0.0 的 Customize 文档开始，检查围绕 IOFileSystem、CacheManager 和 BaseCacheManager 的示例。验证所引用的 API 中哪些可在 2.0.0 中使用，并更新示例使其能够解析；确认文档中的代码片段与该版本发布内容一致。

由索引模型根据 Issue 内容生成。