Corrected install instructions for flake.nix

[TheOddler]

With the old url I got the error

error: unable to download 'https://api.github.com/repos/NorfairKing/feedback/commits/flake': HTTP error 422

       response body:

       {
         "message": "No commit found for SHA: flake",
         "documentation_url": "https://docs.github.com/rest/commits/commits#get-a-commit"
       }

Removing the ref fixes it.