[Datasets] Improve Covost 2
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 