Update example_triangle.md

[ahcorde]

🦟 Dcoumentation fix

Fixed gazebosim/gazebo_test_cases#96

Summary

Update link

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• 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 #555 (5aa8c19) into gz-math7 (9580249) will not change coverage.
The diff coverage is n/a.

❗ Current head 5aa8c19 differs from pull request most recent head 3eaf877. Consider uploading reports for the commit 3eaf877 to get more accurate results

@@            Coverage Diff            @@
##           gz-math7     #555   +/-   ##
=========================================
  Coverage     94.16%   94.16%           
=========================================
  Files           145      145           
  Lines          9774     9774           
=========================================
  Hits           9204     9204           
  Misses          570      570