Fix #20 issue

README.md

@@ -79,7 +79,7 @@ To use with docker you must mount your status file to `/etc/openvpn_exporter/ser
 ```sh
 docker run -p 9176:9176 \
   -v /path/to/openvpn_server.status:/etc/openvpn_exporter/server.status \
-  kumina:openvpn_exporter -openvpn.status_paths /etc/openvpn_exporter/server.status
+  kumina/openvpn-exporter -openvpn.status_paths /etc/openvpn_exporter/server.status
 ```
 
 Metrics should be available at http://localhost:9176/metrics.