Fix broken links in the physics plugin tutorial

[JoanAguilar]

Signed-off-by: Joan Aguilar Mayans [email protected]

🦟 Bug fix

Fixes gazebosim/garden-tutorial-party#333 through gazebosim/garden-tutorial-party#340.

Summary

Updated some links to point to the gz-physics6 branch rather than the (nonexisting) ign-physics6 branch.

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 #417 (2212764) into gz-physics6 (01654fc) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           gz-physics6     #417   +/-   ##
============================================
  Coverage        73.85%   73.85%           
============================================
  Files              128      128           
  Lines             5775     5775           
============================================
  Hits              4265     4265           
  Misses            1510     1510           

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