Update benchmark comparison instructions

[adlarkin]

Signed-off-by: Ashton Larkin ashton@openrobotics.org

Summary

The current instructions for benchmark comparisons point to a file that doesn't exist in ign-gazebo. I have updated the instructions so that users can compare benchmarks if they wish.

I also found that removing the indentation for the code snippets resolves a bug in the README render where ``` would appear at the beginning/end of the code block.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

[codecov]

Codecov Report

Merging #766 (1b93963) into ign-gazebo3 (dfb43a0) will not change coverage.
The diff coverage is n/a.

❗ Current head 1b93963 differs from pull request most recent head 24623e6. Consider uploading reports for the commit 24623e6 to get more accurate results

@@             Coverage Diff              @@
##           ign-gazebo3     #766   +/-   ##
============================================
  Coverage        77.77%   77.77%           
============================================
  Files              214      214           
  Lines            11923    11923           
============================================
  Hits              9273     9273           
  Misses            2650     2650           

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