Update options flag to new double dash format

The nginx exporter logs the following deprecation warning after startup.

```
nginx-prometheus-exporter: the flag format is deprecated and will be removed in a future release, please use the new format: --nginx.scrape-uri
```

This PR intends to fix that deprecated format and use the current flag format with double dashes.

manifests/nginx_prometheus_exporter.pp

@@ -97,7 +97,7 @@
     default => undef,
   }
 
-  $options = "-nginx.scrape-uri '${scrape_uri}' ${extra_options}"
+  $options = "--nginx.scrape-uri '${scrape_uri}' ${extra_options}"
 
   if $install_method == 'url' {
     # Not a big fan of copypasting but prometheus::daemon takes for granted