Fix bash completion script install path

[iche033]

🦟 Bug fix

Summary

Fix bash completion script installation path. Issue reported when building gz-msgs-vendor for rolling:

23:29:16 CMake Error at core/cmd/cmake_install.cmake:82 (file):
23:29:16   file cannot create directory:
23:29:16   /opt/ros/rolling/opt/gz_msgs_vendor/share/gz/gz2.completion.d.  Maybe need
23:29:16   administrative privileges.

updated install path to be consistent with other pacakges, e.g. see install path in gz-transport and sdformat

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

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