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(),
),
);
```

Start with the 2.0.0 Customize documentation and inspect the example around IOFileSystem, CacheManager, and BaseCacheManager. Verify which referenced API is available in 2.0.0 and update the example so it resolves; confirm the documented snippet matches the release.

索引モデルが issue の本文から書いたものです。