Fix type in add_equality_constraint

[ivan-pi]

Probably a mishap from copy/pasting

[codecov]

Codecov Report

Merging #18 (87e2382) into main (7e26398) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #18   +/-   ##
=======================================
  Coverage   29.48%   29.48%           
=======================================
  Files           8        8           
  Lines         563      563           
  Branches      153      153           
=======================================
  Hits          166      166           
  Misses        274      274           
  Partials      123      123           

Impacted Files	Coverage Δ
src/nlopt_wrap.f90	12.85% <ø> (ø)

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[awvwgk]

Good catch.