Updated NATS exporter link (prometheus#748)

I've updated the NATS Exporter to be the one maintained by the NATS team, which was released this week. The previous link was to Lovoo's exporter - Markus (maintainer of Lovoo's) is aware of this change and fine with it.

Author: bjflanne

content/docs/instrumenting/exporters.md

@@ -59,7 +59,7 @@ wide variety of JVM-based applications, for example [Kafka](http://kafka.apache.
 ### Messaging systems
    * [Kafka consumer group exporter](https://github.com/kawamuray/prometheus-kafka-consumer-group-exporter)
    * [Kafka ZooKeeper exporter](https://github.com/cloudflare/kafka_zookeeper_exporter)
-   * [NATS exporter](https://github.com/lovoo/nats_exporter)
+   * [NATS exporter](https://github.com/nats-io/prometheus-nats-exporter)
    * [NSQ exporter](https://github.com/lovoo/nsq_exporter)
    * [RabbitMQ exporter](https://github.com/kbudde/rabbitmq_exporter)
    * [RabbitMQ Management Plugin exporter](https://github.com/deadtrickster/prometheus_rabbitmq_exporter)