docs: fix file extension in package list scanning

[b31ngd3v]

faced this error while i was following instructions from the readme

[codecov-commenter]

Codecov Report

Merging #1733 (a0df3ee) into main (82ce324) will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1733      +/-   ##
==========================================
- Coverage   80.20%   80.05%   -0.16%     
==========================================
  Files         300      300              
  Lines        6427     6427              
  Branches     1050     1050              
==========================================
- Hits         5155     5145      -10     
- Misses       1054     1063       +9     
- Partials      218      219       +1     

Flag	Coverage Δ
longtests	80.05% <ø> (-0.16%)	⬇️

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Impacted Files	Coverage Δ
cve_bin_tool/nvd_api.py	75.20% <0.00%> (-8.80%)	⬇️
cve_bin_tool/cli.py	69.29% <0.00%> (+0.39%)	⬆️

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[terriko]

Looks good to me!

This makes me wonder if we should actually be requiring the .txt extension: we've got the technical ability to check the file type and the extension is only really for human convenience here. But human convenience is important too so there's no harm in it working the way it does. Either way, it's important that the documentation match the actual behaviour, so thank you for this update!