Added mandatory role detail to be able to do this quickstart tutorial

[boletus151]

Some clients have shared that they were unable to complete the tutorial. As the Storage Blob Data Contributor role is mandatory to be able to use the storage account I believe it´s necessary to add this detail to the documentation.
I´ve checked this note is added in other pages, but not here. As it is a requirement to be able to complete the tutorial, I believe it´s important to add it here as well.
Thank you.

[prmerger-automator]

@boletus151 : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[prmerger-automator]

@boletus151 : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit ff0f850:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/ai-services/language-service/custom-text-classification/includes/quickstarts/language-studio.md	✅Succeeded

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[ttorble]

@jboback

Can you review the proposed changes?

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#label:"aq-pr-triaged"
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