Fix bug in reform documentation

[martinholmer]

Non-substantive bug fix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (81bf6b0) to head (fad0746).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2833   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          13       13           
  Lines        2617     2617           
=======================================
  Hits         2602     2602           
  Misses         15       15           

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

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