Fix external union example in union.md

[yukinarit]

Closes #449

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (42d3c5d) 89.59% compared to head (d25d2bd) 89.59%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #455   +/-   ##
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  Coverage   89.59%   89.59%           
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  Files          12       12           
  Lines        1912     1912           
  Branches      397      397           
=======================================
  Hits         1713     1713           
  Misses        145      145           
  Partials       54       54           

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