Fix docstring reference to microdata dictionary name

[jdebacker]

This PR fixes an error in a docstring in taxbrain.py that provided the wrong name for a key if one passes a microdata dictionary to the TaxBrain class object.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.50%. Comparing base (1cd87f4) to head (267b241).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #204   +/-   ##
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  Coverage   79.50%   79.50%           
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  Files          23       23           
  Lines        1947     1947           
=======================================
  Hits         1548     1548           
  Misses        399      399           

Flag	Coverage Δ
unittests	79.50% <ø> (ø)

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