Fix `batch_size` typo for `Stream` object in docs

[snarayan21]

Description of changes:

This addresses a typo where Stream objects had batch_size passed in to them, which is incorrect.

Issue #, if available:

Merge Checklist:

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General

• [ ] I have read the contributor guidelines
• [ ] This is a documentation change or typo fix. If so, skip the rest of this checklist.
• [ ] I certify that the changes I am introducing will be backward compatible, and I have discussed concerns about this, if any, with the MosaicML team.
• [ ] I have updated any necessary documentation, including README and API docs (if appropriate).

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