Update migrating_gazebo_classic_ros2_packages.md

[aryanjagushte]

🦟 Bug fix

Fixes #

Summary

The branch command specified previously, doesn't work.

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.

[azeey]

Thanks! I was hoping it would clone at a specific commit, but the -b command only works with branches.