Fix udocker installation link

[kinow]

Found the broken link when I was going to test #782 👍

[codecov]

Codecov Report

Merging #1667 (1b084cf) into main (ef7cb30) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1667      +/-   ##
==========================================
- Coverage   66.86%   66.86%   -0.01%     
==========================================
  Files          93       93              
  Lines       16622    16622              
  Branches     4414     4414              
==========================================
- Hits        11115    11114       -1     
  Misses       4366     4366              
- Partials     1141     1142       +1     

Impacted Files	Coverage Δ
cwltool/cwltool/job.py	62.64% <0.00%> (-0.20%)	⬇️

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