change the option name in export documentation to "image"

[joelN123]

In the docs, this option is called "images", but when running the export code, an error will come up

Usage: export.py [OPTIONS]
Try 'export.py -h' for help.

Error: no such option: --images

It's because the actual option is "image". (which you can see by running 'export.py -h').
When using "image", it works fine. So this PR is just for correcting the documentation.

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[iizukak]

OA
Thanks.

[iizukak]

/ready

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