[image_picker] Update README example

[MillerAdulu]

Update RetrieveLostDataResponse type to correct LostDataResponse.

Description

The RetrieveLostDataResponse type doesn't exist as part of the standard image_picker library

Related Issues

None

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[MillerAdulu]

I have signed the CLA.

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[cyanglaz]

LGTM! Good catch!
Please also update the CHANGELOG and pubspec with a new version

[MillerAdulu]

Done.