Fix README archive download link

[andreaso]

1. https://github.com/joblib/joblib/archives/master now returns a 404 response
2. It would appear as if GitHub now only provide ZIP archives

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (720ccd9) 94.95% compared to head (d204da5) 94.92%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1509      +/-   ##
==========================================
- Coverage   94.95%   94.92%   -0.03%     
==========================================
  Files          45       45              
  Lines        7550     7550              
==========================================
- Hits         7169     7167       -2     
- Misses        381      383       +2     

see 2 files with indirect coverage changes

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