fix #130 update data preparation guidance for mala_asr

[nuaalixu]

What does this PR do?

Fixes #130

Feature/Issue validation/testing

update data preparation section of README

Doesn't involve testing.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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  Pull Request section?
• Was this discussed/approved via a Github issue? Please add a link
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• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Thanks for contributing 🎉!

[ddlBoJack]

Refer to #168

[yanghaha0908]

Thank you very much for your PR. Based on your content, I have improved the README.md file.