The README.md at the line 86 contains
[pv-centos]: https://github.com/PowerShell/PowerShell/releases/download/v6.1.0-rc.1/powershell-preview-6.1.0-rc.1-1.rhel.7.x86_64.rpm
That link is incorrect - it returns the 404 page.
The link to download the rpm file at the release page is working and is
https://github.com/PowerShell/PowerShell/releases/download/v6.1.0-rc.1/powershell-preview-6.1.0.rc.1-1.rhel.7.x86_64.rpm
The issue is the dot/dash before the rc name part.