performance: fix 4x7 script for latency_probe tests

PERFORMANCE_COUNTER node-count = 4
PERFORMANCE_COUNTER worker-count-per-node = 7
PERFORMANCE_COUNTER actor-count-per-worker = 100
PERFORMANCE_COUNTER worker-count = 28
PERFORMANCE_COUNTER actor-count = 2800
PERFORMANCE_COUNTER message-count-per-actor = 40000
PERFORMANCE_COUNTER message-count = 112000000
PERFORMANCE_COUNTER elapsed-time = 71.158930 s
PERFORMANCE_COUNTER computation-throughput = 1573941.605729 messages / s
PERFORMANCE_COUNTER node-throughput = 393485.401432 messages / s
PERFORMANCE_COUNTER worker-throughput = 56212.200205 messages / s
PERFORMANCE_COUNTER worker-latency = 17789 ns
PERFORMANCE_COUNTER actor-throughput = 562.122002 messages / s ****
PERFORMANCE_COUNTER actor-latency = 1778973 ns

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>