Fix docstring ordering in pane.split_window

[Ngalstyan4]

It seems in 0.29->0.30 pane.split_window() argument order was modified, but the corresponding docstring was not change.
This changes argument ordering in the docstring

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (48ea20b) 88.76% compared to head (d1558c0) 88.38%.
Report is 73 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #528      +/-   ##
==========================================
- Coverage   88.76%   88.38%   -0.38%     
==========================================
  Files          37       37              
  Lines        3685     3797     +112     
  Branches      526      549      +23     
==========================================
+ Hits         3271     3356      +85     
- Misses        281      304      +23     
- Partials      133      137       +4     

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[tony]

@Ngalstyan4 Thanks for the contribution, can you sign the CLA? (In short, to assign copyright of contributions to me ☺️)