Add dbname to example to avoid confusion

README.md

@@ -13,7 +13,7 @@ This package is available for Docker:
 # Start an example database
 docker run --net=host -it --rm -e POSTGRES_PASSWORD=password postgres
 # Connect to it
-docker run --net=host -e DATA_SOURCE_NAME="postgresql://postgres:password@localhost:5432/?sslmode=disable" wrouesnel/postgres_exporter
+docker run --net=host -e DATA_SOURCE_NAME="postgresql://postgres:password@localhost:5432/postgres?sslmode=disable" wrouesnel/postgres_exporter
 ```
 ## Building and running