api: Fix error message
api: Fix error message

Keyspace and table names are separated by a single colon.
Message-Id: <c598af68cc832b0d39d0cff227352b6feda7c13f@scylladb.com>

duarten/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla,kjniemi/scylla,raphaelsc/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla