Fix benign bug in midpoint evaluation.

[f0uriest]

Resolves #16

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.83%. Comparing base (48fe56b) to head (fa14060).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #17   +/-   ##
=======================================
  Coverage   93.83%   93.83%           
=======================================
  Files           7        7           
  Lines         616      616           
=======================================
  Hits          578      578           
  Misses         38       38           

Files with missing lines	Coverage Δ
quadax/adaptive.py	96.87% <100.00%> (ø)
quadax/romberg.py	100.00% <100.00%> (ø)