Fixed logging driver compose example

docs/firststeps-logging.md

@@ -35,7 +35,7 @@ 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"  
 ```