Fix multi_lrauv_race example

[caguero]

Fix the multi_lrauv_race example. In particular I found two issues:

1. The topic to control the thruster of the vehicles needs to be updated.
2. I'm not 100% sure about this one but the turbidity generator looks very dark in my machine. Maybe @mabelzhang can confirm if that's the expected behavior. For now I just removed it to visualize a more clear race.

With turbidity:

No turbidity:

Summary

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 #2111 (899dd99) into gz-sim8 (ecc91e8) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 899dd99 differs from pull request most recent head 85ebb67. Consider uploading reports for the commit 85ebb67 to get more accurate results

@@             Coverage Diff             @@
##           gz-sim8    #2111      +/-   ##
===========================================
+ Coverage    65.45%   65.47%   +0.01%     
===========================================
  Files          323      323              
  Lines        30699    30699              
===========================================
+ Hits         20095    20100       +5     
+ Misses       10604    10599       -5     

see 2 files with indirect coverage changes

[mabelzhang]

Haha, no that's not the expected behavior. Pitch dark very nice for deep sea though.
I've noticed it broken some time back but wasn't sure what changed in the particle emitter interface. Thanks for fixing it!

[iche033]

ticketed an issue for the particle bug: gazebosim/gz-rendering#902