Fix typo plugins to plugin directory in README.md

[avanmalleghem]

🦟 Bug fix

Summary

Update README;md for custom sensor system to fix typo "plugins" -> "plugin"

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.

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🎈 Release

Preparation for <X.Y.Z> release.

Comparison to <x.y.z>: https://github.com/gazebosim//compare/...

Needed by <PR(s)>

Checklist

• [ ] Asked team if this is a good time for a release
• [ ] There are no changes to be ported from the previous major version
• [ ] No PRs targeted at this major version are close to getting in
• [ ] Bumped minor for new features, patch for bug fixes
• [ ] Updated changelog
• [ ] Updated migration guide (as needed)
• [ ] Link to PR updating dependency versions in appropriate repository in gazebo-release (as needed):

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.

[ahcorde]

@osrf-jenkins run tests please!