[hotfix][doc] add single quotes to csv ddl property

[bowenli86]

What is the purpose of the change

add single quotes to csv ddl property

Brief change log

add single quotes to csv ddl property

Verifying this change

n/a

Does this pull request potentially affect one of the following parts:

n/a

Documentation

n/a

[bowenli86]

@wuchong @JingsongLi

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Last check on commit e36fe5f (Wed Dec 18 01:41:31 UTC 2019)

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[wuchong]

Thanks. I will merge it.