types/scylla-sstable-index: update URL to cql3-type-mapping.md
types/scylla-sstable-index: update URL to cql3-type-mapping.md

Said document was recently moved but the URL was not updated.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20210120083816.37774-1-a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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