Fixed category for uptime graph.

plugins/uptime

@@ -2,6 +2,7 @@ config_uptime() {
   echo "graph_title Uptime"
   echo "graph_args --base 1000 -l 0 "
   echo "graph_vlabel uptime in days"
+  echo "graph_category system"
   echo "uptime.label uptime"
   echo "uptime.draw AREA"
 }