changed references to singularity .simg files to .sif files

[davidecarlson]

Per issue #12, this PR changes references in the installation instruction from singularity .simg files to the more current and standard .sif file format.

This should not have any other effect on the singularity image build process itself. I was able to successfully build the alphafold3.sif file.

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[mlbileschi]

Thank you! Interesting -- it looks like this is only a rename -- is the file extension load-bearing? As in does it serialize differently based on the file extension? Or is this cosmetic?

[davidecarlson]

I think that with relatively recent versions of singularity or apptainer (e.g., 3.0 and later), the difference should only be cosmetic because the .simg format isn't used at all.

[mlbileschi]

Thanks a bunch! merged in https://github.com/google-deepmind/alphafold3/commit/e07682c0643881390f59a7696fd91ad0232ea1ea