Fix crucial references to withdrawal

[nielstron]

This most definitely broke when merging #312 without properly checking the results of the merged outcome.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (b2d74cc) 83.11% compared to head (b619174) 83.11%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
pycardano/txbuilder.py	33.33%	2 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #316   +/-   ##
=======================================
  Coverage   83.11%   83.11%           
=======================================
  Files          28       28           
  Lines        3441     3441           
  Branches      844      844           
=======================================
  Hits         2860     2860           
  Misses        430      430           
  Partials      151      151           

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