Fix image save example

[hristog]

Description

This fixes the Image.save() example (see original listing in the appendix below) by actually calling the save method.
The example doesn't seem to work, otherwise, due to the image object not being iterable.

Reference version:

>>> import docker
>>> docker.__version__
'4.3.0-dev'

Appendix A: Original example listing

        Example:
            >>> image = cli.get_image("busybox:latest")
            >>> f = open('/tmp/busybox-latest.tar', 'wb')
            >>> for chunk in image:
            >>>   f.write(chunk)
            >>> f.close()