Fix broken doctest on broken_chains()

[JoelPasvolsky]

CircleCI is showing just this failure but I might need to fix another that shows up locally (or it might be a difference in Python versions)
This is to deal with the doctest failure in this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.93%. Comparing base (edaef27) to head (174e063).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #586      +/-   ##
==========================================
- Coverage   89.33%   86.93%   -2.41%     
==========================================
  Files          22       22              
  Lines        1997     1997              
==========================================
- Hits         1784     1736      -48     
- Misses        213      261      +48     

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