Fix typo in organizations.rb

[areitz-stripe]

Fix a small typo in a comment in lib/octokit/client/organizations.rb.

Additional info

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Added the appropriate label for the given change

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• No

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• Bugfix: Type: Bug
• Feature/model/API additions: Type: Feature
• Updates to docs or samples: Type: Documentation
• Dependencies/code cleanup: Type: Maintenance

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[kfcampbell]

Thanks!