Fix docs for Volume User= and Group= options

[matthijskooijman]

These are not options directly passed to --opt, but these use --opt o=, so update the table accordingly.

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Does this PR introduce a user-facing change?

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[baude]

LGTM

[packit-as-a-service]

[NON-BLOCKING] Packit jobs failed. @containers/packit-build please check. Everyone else, feel free to ignore.

[Luap99]

LGTM