docs(python): Fix substr function param definition

[ZemanOndrej]

#19053

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.77%. Comparing base (e186c03) to head (d237dd8).
Report is 552 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #19054   +/-   ##
=======================================
  Coverage   79.77%   79.77%           
=======================================
  Files        1531     1531           
  Lines      208487   208487           
  Branches     2913     2913           
=======================================
+ Hits       166314   166322    +8     
+ Misses      41622    41614    -8     
  Partials      551      551           

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[mcrumiller]

@ZemanOndrej do a merge/rebase with main to fix the failing test.

[lukemanley]

This PR seems to have gone stale but looks like a good fix to the docs.

[ritchie46]

Thanks!