Fix help: Set-GitHubConfiguration -DefaultRepositoryName

[matt9ucci]

Description

Would be a copy-paste error from -DefaultOwnerName.

Checklist

• [ ] You actually ran the code that you just wrote, especially if you did just "one last quick change".
• [x] Comment-based help added/updated, including examples.
• [ ] Static analysis is reporting back clean.
• [ ] New/changed code adheres to our coding guidelines.
• [ ] Formatters were created for any new types being added.
• [ ] New/changed code continues to support the pipeline.
• [ ] Changes to the manifest file follow the manifest guidance.
• [ ] Unit tests were added/updated and are all passing. See testing guidelines. This includes making sure that all pipeline input variations have been covered.
• [ ] Relevant usage examples have been added/updated in USAGE.md.
• [ ] If desired, ensure your name is added to our Contributors list