examples/perf: use 1 connection per shard

The `perf` example has demonstrated an antipattern of using 1 connection
per host, which is not recommended for production use in case of
ScyllaDB. This commit updates the example to use 1 connection per shard,
which is the recommended practice for ScyllaDB.

Author: wprzytula

examples/perf/perf.c

@@ -137,7 +137,7 @@ CassCluster* create_cluster(const char* hosts) {
   cass_cluster_set_credentials(cluster, "cassandra", "cassandra");
   cass_cluster_set_num_threads_io(cluster, NUM_IO_WORKER_THREADS);
   cass_cluster_set_queue_size_io(cluster, 10000);
-  cass_cluster_set_core_connections_per_host(cluster, 1);
+  cass_cluster_set_core_connections_per_shard(cluster, 1);
   return cluster;
 }