Update command_reference.md

[sumengzs]

更新bench --small-file-size 参数单位和默认大小 与实际 juicefs 参数不一致问题

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✅ sumengzs
❌ sumeng

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[SandyXSD]

@sumengzs Could you please fix it on the English documentation as well? Also the CLA need to be signed before merging the PR.

[sumengzs]

@sumengzs你能在英文文档上也修复这个问题吗？此外，在合并 PR 之前需要签署 CLA。

ok