Merge pull request #1085 from lucperkins/lperkins/issue-1080-javadoc-link

Fix Javadoc link to PushGateway class

content/docs/instrumenting/pushing.md

@@ -16,7 +16,7 @@ makes it easy to instrument even shell scripts without a client library.
 [README.md](https://github.com/prometheus/pushgateway/blob/master/README.md).
 
  * For use from Java see the
-[PushGateway](https://prometheus.io/client_java/io/prometheus/client/exporter/PushGateway.html)
+[PushGateway](https://prometheus.github.io/client_java/io/prometheus/client/exporter/PushGateway.html)
 class.
 
  * For use from Go see the [Push](http://godoc.org/github.com/prometheus/client_golang/prometheus#Push) and [PushAdd](http://godoc.org/github.com/prometheus/client_golang/prometheus#PushAdd) functions.