kubevirtbot referencing CONTRIBUTING.md that does no longer exist

[kastl-ars]

Dear maintainers,

when kubevirtbot complains about missing DCO, it links to a non-existent file (and uses master instead of main):

📝 Please follow instructions in the [contributing guide](https://github.com/kubevirt/containerdisks/blob/master/CONTRIBUTING.md) to update your commits with the DCO

Kind Regards,
Johannes