Fix broken link in MassMatrix3.hh

[adlarkin]

Signed-off-by: Ashton Larkin [email protected]

🦟 Bug fix

Fixing a broken link that was noticed in #195 (review).

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

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[codecov]

Codecov Report

Merging #197 (6f4dcfa) into ign-math6 (19f9372) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-math6     #197   +/-   ##
==========================================
  Coverage      99.20%   99.20%           
==========================================
  Files             63       63           
  Lines           6020     6020           
==========================================
  Hits            5972     5972           
  Misses            48       48           

Impacted Files	Coverage Δ
include/ignition/math/MassMatrix3.hh	99.40% <ø> (ø)

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