fix "rbg" -> "rgb" in image_dataset_from_directory

[YoniChechik]

in image_dataset_from_directory in field color_mode in case of ValueError there was a typo where it said "rbg" instead of "rgb".

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[YoniChechik]

@googlebot I signed it!

[YoniChechik]

I don't really understand this error in the checks: import/copybara — An error happened while migrating the change. Is there anything I need to do @fchollet?

[fchollet]

Is there anything I need to do

No, that's an issue we'll fix on our side.