README: fix a dead link for building instructions
README: fix a dead link for building instructions

The link was outdated, since its destination was moved to
a subdirectory.

Message-Id: <559b12e3a633ac4b7ff6419b21f251b49381865c@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla