fixed the broken link docker-compose.yaml in README.md of prometheus-grafana (#152)

Signed-off-by: Dinesh kumar <dineshkumar12004@gmail.com>

prometheus-grafana/README.md

@@ -12,7 +12,7 @@ Project structure:
 └── README.md
 ```
 
-[_docker-compose.yaml_](docker-compose.yaml)
+[_docker-compose.yml_](docker-compose.yml)
 ```
 version: "3.7"
 services: