HACKING.md: Update "Contributing" URL
HACKING.md: Update "Contributing" URL

The old page results in a 404 error.

duarten/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla