Couple of debian fixes, cannot use variable expansions this way

README.md

@@ -32,11 +32,11 @@ sudo systemctl start bird-exporter
 
 Default configuration:
 ```
-BIRD_RUN_USER=bird
-BIRD_RUN_GROUP=bird
-BIRD_EXPORTER_ARGS="-period=60s -bird.socket=/var/run/bird/bird.ctl"
+BIRD_EXPORTER_ARGS="-web.listen-address=:9324 -period=60s -bird.socket=/var/run/bird/bird.ctl"
 ```
 
+The service runs as the `bird` user and group.
+
 ## Documentation
 
 - **Manual page**: `man bird-exporter` (after package installation)