Update annotated_images.show_sample() example

[rxcoulton]

Is your documentation update related to a problem? Please describe.
Example in docs is wrong: https://mapreader.readthedocs.io/en/latest/User-guide/Classify/Train.html

Describe the solution you'd like
correct .show_image_labels() code snipped to .show_sample()

Describe alternatives you've considered

Additional context
discussed with Rosie during the training session

[rwood-97]

Thanks, will add this as part of #417