save_png needs improved explanation in CLI docs

[agrimwood]

Command Line Tools

## Predict

Optional Arguments:

--save_png example usage seems contradictory, readthedocs states:

--save_png, for saving the outputs in png format.

--save_png, for not saving the outputs in png format.

If the documentation request is approved, would you be willing to submit a PR? (Help
can be provided if you need assistance submitting a PR)

Yes

[mathpluscode]

ooops it should be --no-save_png :(

[mathpluscode]

I handled it in PR #568

[mathpluscode]

fixed in #568

see https://deepreg.readthedocs.io/en/latest/docs/cli.html#predict