Update .coveragerc to include dfifpy.snmf instead of src

[bobleesj]

Crosschecked with diffpy.utils - https://github.com/diffpy/diffpy.utils/blob/main/.coveragerc

and locally codecov works.

                                 Stmts   Miss  Cover
----------------------------------------------------
diffpy/snmf/__init__.py              2      0   100%
diffpy/snmf/containers.py           22      0   100%
diffpy/snmf/factorizers.py           6      0   100%
diffpy/snmf/io.py                   33     33     0%
diffpy/snmf/optimizers.py           13      0   100%
diffpy/snmf/polynomials.py          12      0   100%
diffpy/snmf/stretchednmfapp.py      24     24     0%
diffpy/snmf/subroutines.py         126     11    91%
----------------------------------------------------
                                   238     68    71%

[github-actions]

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