Fix specification of domestic production deduction under TCJA

[martinholmer]

Fixes #2798.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (c698a38) to head (14406e9).
Report is 15 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2807   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          13       13           
  Lines        2614     2614           
=======================================
  Hits         2599     2599           
  Misses         15       15           

Flag	Coverage Δ
unittests	99.42% <100.00%> (ø)

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Files with missing lines	Coverage Δ
taxcalc/policy.py	100.00% <100.00%> (ø)