Fix grammar in README.md description

Author: joecorall

README.md

@@ -2,7 +2,7 @@
 
 [ops-agent](https://docs.cloud.google.com/stackdriver/docs/solutions/agents/ops-agent) metrics for Google Compute Engine instances running Google Container Optimized OS.
 
-This service scrapes cadvisor every 30s. filters out metrics, and sends the unfiltered metrics to Google Cloud Monitoring
+This service scrapes cadvisor every 30s, filters out metrics, and sends the unfiltered metrics to Google Cloud Monitoring
 
 ## Attribution