[Release pipeline] Preview release notes names have inconsistent spacing

[ellahathaway]

In our release notes for preview releases (preview & rc), there's not a space after RC but there is after Preview: .NET X.0 RCY vs .NET X.0 Preview Y. The .NET Core release notes use a space for RC: https://github.com/dotnet/core/blob/main/release-notes/9.0/preview/rc1/9.0.0-rc.1.md, so we should do that too. That'll make things consistent between Preview and RC.

[ellahathaway]

This will be closed by https://dev.azure.com/dnceng/internal/_git/dotnet-release/pullrequest/51571

[ellahathaway]

PR merged, closing