README.md fix test split size 500TB->500GB

[ziky90]

Why?

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What is the use case?

How ?

Document the technical decisions you have made.
If some parts are WIP, please mention them here.

Test plan

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• Was the content of this PR discussed and approved via a GitHub issue? If so, it fixes #{issue number} (no need for typos or documentation improvements)
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• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?

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[ziky90]

README typo fix according to #3

[zyaoj]

@ziky90 Thanks!