Minor Doxygen fixes

[darbyjohnston]

This PR fixes a couple of minor Doxygen typos that were causing warnings.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.13%. Comparing base (2429dd5) to head (72836f3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1984   +/-   ##
=======================================
  Coverage   85.13%   85.13%           
=======================================
  Files         181      181           
  Lines       12783    12783           
  Branches     1206     1206           
=======================================
  Hits        10883    10883           
  Misses       1717     1717           
  Partials      183      183           

Flag	Coverage Δ
py-unittests	85.13% <ø> (ø)

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Files with missing lines	Coverage Δ
src/opentime/rationalTime.h	95.83% <ø> (ø)
src/opentimelineio/serializableObject.h	83.67% <ø> (ø)

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