Ensure EITC phaseout uses the correct rate

[nikhilwoodruff]

Adding any fixes here that I find while testing the US earnings chart.

[codecov]

Codecov Report

Merging #618 (35b1c0e) into master (8535b6c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 35b1c0e differs from pull request most recent head 9bde836. Consider uploading reports for the commit 9bde836 to get more accurate results

@@           Coverage Diff           @@
##           master     #618   +/-   ##
=======================================
  Coverage   96.11%   96.11%           
=======================================
  Files         237      237           
  Lines        5304     5306    +2     
  Branches      571      571           
=======================================
+ Hits         5098     5100    +2     
  Misses        178      178           
  Partials       28       28           

Impacted Files	Coverage Δ
...iables/irs/credits/earned_income/eitc_reduction.py	100.00% <100.00%> (ø)

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