Fix incorrect --output argument in generate image example

[ernilambar]

Closes #18

The inline example used --output which is not a registered argument. The correct argument is --destination-file.

- $ wp ai generate image "A minimalist WordPress logo" --output=wp-logo.png
+ $ wp ai generate image "A minimalist WordPress logo" --destination-file=wp-logo.png

[github-actions]

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[codecov]

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