Doc: update the alter table help command content

[EcaleD]

fix #55

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Change logs

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• Removed the 'FOR (RANK(number))' option since this was not supported
• Updated format of the 'ALTER PARTITION' syntax

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[tuhaihe]

Do we need to update the corresponding part on our website? @EcaleD

[EcaleD]

Do we need to update the corresponding part on our website? @EcaleD

Just checked the doc here, and it looks correct.