Trivial Fuel URL fix in example

[mabelzhang]

🦟 Bug fix

I broke my Gazebo Fuel accounts. The authorities fixed the mess for me. But that means I’ve consolidated my accounts, and there’s an URL change to model files under my account. Thankfully it only affects one SDF file that only exists from ign-gazebo6 onwards.

Sorry to anyone using it. My fault for not knowing how to Internet.

Summary

Just make sure the models show up:

ign gazebo src/ign-gazebo/examples/worlds/multi_lrauv_race.sdf 

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 #1479 (239702b) into ign-gazebo6 (52d12f1) will not change coverage.
The diff coverage is n/a.

❗ Current head 239702b differs from pull request most recent head b054d59. Consider uploading reports for the commit b054d59 to get more accurate results

@@             Coverage Diff              @@
##           ign-gazebo6    #1479   +/-   ##
============================================
  Coverage        33.58%   33.58%           
============================================
  Files               44       44           
  Lines             2260     2260           
============================================
  Hits               759      759           
  Misses            1501     1501           

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