changed image1 to moving_image in tvl1 parameter docs

[cwood1967]

Description

The docstrings in skimage.registration._optical_flow.py for optical_flow_tvl1 and _tvl1 both have the parameter num_warp described as Number of times image1 is warped. This is confusing since image1 could be either the reference_image or the moving_image. The _ilk docstrings have moving_image rather than image1, so this change would make it consistent.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
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• Unit tests
• Clean style in the spirit of PEP8
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[alexdesiqueira]

Thank you @cwood1967, nice catch! Likely possible that at some point the parameters were image0, image1 and we forgot to update the docs 😅
Thanks again!