monitoring: fix usage in README
monitoring: fix usage in README

Change-Id: I198ca59e8d6e6ff55e878f7996223e120c4feee8

adamyanliev/golang-samples,campoy/golang-samples,GoogleCloudPlatform/golang-samples,adamyanliev/golang-samples,adamyanliev/golang-samples,GoogleCloudPlatform/golang-samples,campoy/golang-samples,danicat/golang-samples,campoy/golang-samples,GoogleCloudPlatform/golang-samples,danicat/golang-samples,adamyanliev/golang-samples,danicat/golang-samples