Fixed logging driver compose example

docs/firststeps-logging.md

@@ -35,9 +35,9 @@ Edit `docker-compose.yml` and append, for example, this block to use the "gelf"
 
 ```
 logging:
-  log_driver: "gelf"
+  driver: "gelf"
   options:
-    gelf-address: "udp://graylog:12201"
+    gelf-address: "udp://graylog:12201"  
 ```
 
 Linux users can also add or edit the Docker daemons configuration file `/etc/docker/daemon.json` to affect the global logging behavior. Windows users please have a look at the [docker documentation](https://docs.docker.com/engine/reference/commandline/dockerd//#windows-configuration-file):