[Datasets] Improve Covost 2
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I am trying to use `load_dataset` with the French dataset(common voice corpus 1) which is downloaded from a common voice site and the target language is English (using colab)

Steps I have followed:

**1. untar:**
`!tar xvzf fr.tar -C data_dir`

**2. load data:**
`load_dataset('covost2', 'fr_en', data_dir="/content/data_dir")`

0 rows are loading as shown below:
```
Using custom data configuration fr_en-data_dir=%2Fcontent%2Fdata_dir
Reusing dataset covost2 (/root/.cache/huggingface/datasets/covost2/fr_en-data_dir=%2Fcontent%2Fdata_dir/1.0.0/bba950aae1ffa5a14b876b7e09c17b44de2c3cf60e7bd5d459640beffc78e35b)
100%
3/3 [00:00<00:00, 54.98it/s]
DatasetDict({
    train: Dataset({
        features: ['client_id', 'file', 'audio', 'sentence', 'translation', 'id'],
        num_rows: 0
    })
    validation: Dataset({
        features: ['client_id', 'file', 'audio', 'sentence', 'translation', 'id'],
        num_rows: 0
    })
    test: Dataset({
        features: ['client_id', 'file', 'audio', 'sentence', 'translation', 'id'],
        num_rows: 0
    })
})
```

Can you please provide a sample working example code to load the dataset? Hi ! I think it only works with the subsets of Common Voice Corpus 4, not Common Voice Corpus 1
It's currently quite confusing to understand the manual data download instruction of Covost and not very user-friendly.

Currenty the user has to:

1. Go on Common Voice website
2. Find the correct dataset which is **not** mentioned in the error message
3. Download it
4. Untar it
5. Create a language id folder (why? this folder does not exist in the `.tar` downloaded file)
6. pass the folder containing the created language id folder

This PR improves this to:

1. Go on Common Voice website
2. Find the correct dataset which **is** mentioned in the error message
3. Download it
4. Untar it
5. pass the untared folder

**Note**: This PR is not at all time-critical 
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