Default config for squad is incorrect
### Describe the bug

If you download Squad, it will download the plain_text version, but the config still specifies "default", so if you set the offline mode the cache will try to look it up according to the config_id which is "default" and this will say;
ValueError: Couldn't find cache for squad for config 'default'
Available configs in the cache: ['plain_text']

### Steps to reproduce the bug

1. export HF_DATASETS_OFFLINE=0
2. load_dataset("squad")
3. export HF_DATASETS_OFFLINE=1
4. load_dataset("squad")

### Expected behavior

We should change the config_name I guess? 

### Environment info

linux, latest version of datasets