add total usage pie chart based on total/free bytes

Author: harshavardhana

docs/metrics/prometheus/grafana/README.md

@@ -11,7 +11,7 @@
 
 Visualize MinIO metrics with our official Grafana dashboard available on the [Grafana dashboard portal](https://grafana.com/grafana/dashboards/13502).
 
-Refer to the dashboard [json file here](https://raw.githubusercontent.com/minio/minio/master/docs/metrics/prometheus/grafana/minio-overview.json).
+Refer to the dashboard [json file here](https://raw.githubusercontent.com/minio/minio/master/docs/metrics/prometheus/grafana/minio-dashboard.json).
 
 ![Grafana](https://raw.githubusercontent.com/minio/minio/master/docs/metrics/prometheus/grafana/grafana-minio.png)