chore: add contact address to Security policy doc

[milancermak]

In the light of the recent Vyper issue, I was checking if this repo has a safe and trusted way to report security issues. The good thing is there is a Security policy document. The bad thing is that the "private contract address" link in it does not actually contain a way how to privately and securely contact the Cairo compiler team. Please fix it to allow for responsible security disclosures.