docs(python): correct default in rolling_* function examples

[MarcoGorelli]

The docs examples state

Compute the rolling var with the default left closure of temporal windows

although closed='right' has been the default for a while

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.91%. Comparing base (075817b) to head (3daf449).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16000      +/-   ##
==========================================
+ Coverage   80.88%   80.91%   +0.02%     
==========================================
  Files        1384     1384              
  Lines      178013   178112      +99     
  Branches     3043     3043              
==========================================
+ Hits       143991   144112     +121     
+ Misses      33539    33518      -21     
+ Partials      483      482       -1     

Flag	Coverage Δ
python	74.39% <ø> (+0.02%)	⬆️
rust	78.04% <ø> (+0.02%)	⬆️

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