Fix setup.cfg name

[C-Achard]

Fixes warning when building from setuptools :

    ********************************************************************************
    Usage of dash-separated 'napari-cellseg3d' will not be supported in future
    versions. Please use the underscore name 'napari_cellseg3d' instead.
    By 2024-Sep-26, you need to update your project and remove deprecated calls
    or your builds will no longer be supported.
    See https://setuptools.pypa.io/en/latest/userguide/declarative_config.html for details.
    ********************************************************************************

[C-Achard]

@MMathisLab Just to confirm, this will not affect the project name on PyPi, correct ?
I tried on test-pypi (https://test.pypi.org/project/napari-cellseg3d/) manually with twine, it seems to upload and then install fine, I just want to make sure switching to napari_cellseg3d instead of napari-cellseg3d will not try to upload to a different PyPi project

[MMathisLab]

this I don't know, but suspect fine -- if you can push it then I think its okay