Docs: Fix wrong description of 'format'

[sinbai]

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Description

Per the code, format should be optional and default to xml. Updated documentation to fix this issue found in #26959.

This is a (please select all that apply):

• Bug Fix
• New Feature (ie adding a service, resource, or data source)
• Enhancement
• Breaking Change

Related Issue(s)

Fixes #26959

[stephybun]

Thanks @sinbai LGTM 👍

[github-actions]

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