Fix --tyro-write-completion when use_underscores=True

[brentyi]

Fixes #172

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.70%. Comparing base (8f2c05a) to head (73b4b79).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #173   +/-   ##
=======================================
  Coverage   99.70%   99.70%           
=======================================
  Files          24       24           
  Lines        2042     2042           
=======================================
  Hits         2036     2036           
  Misses          6        6           

Flag	Coverage Δ
unittests	99.70% <ø> (ø)

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