DOC: Update description of the Cauchy cdf and its complement.

[WarrenWeckesser]

Also add a missing closing parenthesis in the pdf expression.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.83%. Comparing base (2f3c798) to head (742b5cc).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1237   +/-   ##
========================================
  Coverage    93.83%   93.83%           
========================================
  Files          657      657           
  Lines        55244    55244           
========================================
  Hits         51840    51840           
  Misses        3404     3404           

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