tutorials/09_use_custom_engine: Add actual build&run instructions

[peci1]

🦟 Bug fix

Fixes gazebosim/gazebo_test_cases#1428

Summary

The tutorial included wrong instructions for building the example. This PR fixes that.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 79.44%. Comparing base (b181d63) to head (1a02224).
Report is 8 commits behind head on gz-physics8.

Additional details and impacted files

@@             Coverage Diff              @@
##           gz-physics8     #689   +/-   ##
============================================
  Coverage        79.44%   79.44%           
============================================
  Files              144      144           
  Lines             8646     8646           
============================================
  Hits              6869     6869           
  Misses            1777     1777           

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[azeey]

Looks like there are some CI issues, but I'll go ahead and merge this since it's only updating documentation.