Update example_vaccine.py

[Jimazcoder]

modified causal test case and linear regression estimator to fit class attributes

[jmafoster1]

Thanks @Jimazcoder, our tests should have picked this up.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.84%. Comparing base (192cfca) to head (cdbaf1c).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #322   +/-   ##
=======================================
  Coverage   93.84%   93.84%           
=======================================
  Files          27       27           
  Lines        1593     1593           
=======================================
  Hits         1495     1495           
  Misses         98       98           

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