docs: Added rel path to --out-dir example

[rimutaka]

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  > not applicable, it's a minor docs fix

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This PR adds relative path examples to --out-dir docs section.

The current wording does not make it clear if the path is relative to the current directory or the root of the package.
The proposed clarification and the examples should make it less ambiguous and save the users from figuring it out by trial and error.