Fix systemd example for configuring nginx_exporter.service (#650)

Author: bruno-espino

examples/systemd/nginx_exporter.service

@@ -4,7 +4,7 @@ Requires=nginx_exporter.socket
 
 [Service]
 User=nginx_exporter
-ExecStart=/usr/local/bin/nginx-prometheus-exporter --nginx.scrape-uri="http://127.0.0.1:8080/stub_status" --web.systemd-socket
+ExecStart=/usr/local/bin/nginx-prometheus-exporter --nginx.scrape-uri=http://127.0.0.1:8080/stub_status --web.systemd-socket
 
 [Install]
 WantedBy=multi-user.target