Fix import command syntax for GitHub custom property documentation

[deiga]

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Before the change?

• The import docs example is showing a broken syntax due to treating < as HTML tags (probably): $ terraform import github_repository_custom_property.example ::

After the change?

• The import example should show up correctly: $ terraform import github_repository_custom_property.example organization-name:repo-name:custom-property-name

Pull request checklist

• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[nickfloyd]

❤️