minor README doc fix re. collector.perf.cpus
minor README doc fix re. collector.perf.cpus

Signed-off-by: Michael Vorburger <a17fed27eaa842282862ff7c1b9c8395a26ac320@vorburger.ch>

siavashs/node_exporter,prometheus/node_exporter,siavashs/node_exporter,siavashs/node_exporter,prometheus/node_exporter,prometheus/node_exporter,siavashs/node_exporter