docs(readme): update help output example

README.md

@@ -62,9 +62,12 @@ macos-battery-exporter -h
 ```
 
 ```
-Usage of macos-battery-exporter:
+usage: macos-battery-exporter [<options>]
+
   -b string
     	Bind address to run server on (default "127.0.0.1")
+  -d string
+    	Log output device (stderr or stdout) (default "stderr")
   -l string
     	Log level (default "info")
   -n string