Change the prometheus example to use prometheus_url (#2790)

The example prometheus config is not using the correct parameter name.

prometheus/datadog_checks/prometheus/data/conf.yaml.example

@@ -5,7 +5,7 @@ instances:
   ## @param prometheus_url - string - required
   ## The URL where your application metrics are exposed by Prometheus.
   #
-  - prometheus_endpoint: http://localhost:10055/metrics
+  - prometheus_url: http://localhost:10055/metrics
 
   ## @param namespace - string - required
   ## The namespace to be appended before all metrics namespace