Small fixed in doxygen

[ahcorde]

Signed-off-by: Alejandro Hernández [email protected]

🦟 Bug fix

Summary

Minor doxygen fixes

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #355 (568f927) into ign-math6 (f2b462d) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-math6     #355   +/-   ##
==========================================
  Coverage      99.65%   99.65%           
==========================================
  Files             67       67           
  Lines           6380     6380           
==========================================
  Hits            6358     6358           
  Misses            22       22           

Impacted Files	Coverage Δ
include/ignition/math/Quaternion.hh	100.00% <ø> (ø)

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[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-03-01-citadel-edifice-fortress/1313/1