"password_override" option in redshift operator.

[skokado]

I' sorry for opened a same content issue in #1328

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I found an issue in document, password_override option of redshift operator.

redshift>: Redshift operations

• Name of the secret key used by default

-> aws.redshift.password, not redshift.password

• Description of how to specify password_override value in option document

-> In case below,

password_override: another_password

it will be used aws.redshift.another_password secret key actually. (I checked this in v0.9.40)
In document, it is written as redshift.another_password.

[chezou]

@skokado Thank you for your contribution, but I will close this PR since the document should be managed in digdag repository.

Can you modify this file and create a PR at digdag repository not here, digdag-docs repo?