Moving dataset between namespaces breaks dataset viewer
## Describe the bug
I moved a dataset from my own namespace to an org and that broke the dataset viewer. To fix it I had to manually edit the `dataset_info.json` file and change the first key in the json from `username--datasetname` to `orgname--datasetname`

## Steps to reproduce the bug
What I did was: 
1- Upload a dataset to my own namespace using `push_to_hub`
2- Move the dataset from my namespace to an org using the web interface.

## Expected results
For the file to be changed accordingly.

## Actual results
Broken dataset viewer.

## Environment info
- `datasets` version: 2.3.3.dev0
- Platform: Linux-4.15.0-189-generic-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.5
- PyArrow version: 7.0.0
- Pandas version: 1.3.5

Moving dataset between namespaces breaks dataset viewer: ## Describe the bug
I moved a dataset from my own namespace to an org and that broke the dataset viewer. To fix it I had to manually edit the `dataset_info.json` file and change the first key in the json from `username--datasetname` to `orgname--datasetname`

## Steps to reproduce the bug
What I did was: 
1- Upload a dataset to my own namespace using `push_to_hub`
2- Move the dataset from my namespace to an org using the web interface.

## Expected results
For the file to be changed accordingly.

## Actual results
Broken dataset viewer.

## Environment info
- `datasets` version: 2.3.3.dev0
- Platform: Linux-4.15.0-189-generic-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.5
- PyArrow version: 7.0.0
- Pandas version: 1.3.5
