Update doc about EROFS support

### What is the version of your Accelerated Container Image

_No response_

### What would you like to be added?

Since erofs conversion has been supported for a while. Please update the document if you have time～
https://github.com/containerd/accelerated-container-image/blob/main/docs/MULTI_FS_SUPPORT.md
@hsiangkao

### Why is this needed for Accelerated Container Image?

update doc

### Are you willing to submit PRs to contribute to this feature?

- [ ] Yes, I am willing to implement it.