Copy-paste typo fix io_utils.py

[fcolecumberri]

I think you copied the first part of text_dataset_from_directory doc into image_dataset_from_directory doc and forgot to change the text to image.

You might also want to update the webpage after this.

https://autokeras.com/utils/

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[haifeng-jin]

Thank you!

[haifeng-jin]

@fcolecumberri You may need to sign the Google CLA before the PR could be merged.

[fcolecumberri]

@haifeng-jin I have already signed the Google CLA