Tweak error message in provider_utils.go

[kfcampbell]

This resolves a small inaccuracy in an error message I ran into while debugging earlier in the week.

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Behavior

Before the change?

Regardless of whether the error was with the GITHUB_OWNER or GITHUB_ORGANIZATION, the error message reported the issue was with the GITHUB_OWNER environment variable.

After the change?

The error message reads fmt.Errorf("configured owner %q is a user, not an organization", meta.(*Owner).name).

Other information

Similar to #1394, this is a small thing that's been bothering me.

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Additional info

Pull request checklist

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

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes (Please add the Type: Breaking change label)
• No

If Yes, what's the impact:

• N/A

Pull request type

Please add the corresponding label for change this PR introduces:

• 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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