Modified nvidia_gpu_by_user plugin's usage

plugins/gpu/nvidia_gpu_by_user

@@ -17,13 +17,14 @@ Add this to node configuration file:
 
 =head1 USAGE
 
+If env.gpuusers is set, graph always shows listed users
+(root, user1, user2 in example below) whether using GPU or not.
+Otherwise, graph shows users that are using the GPU right now only.
+
 Example:
  [nvidia_gpu_by_user]
   env.smiexec /path/to/nvidia-smi
-  env.gpuusers root hideki
-
-If env.gpuusers is set, graph always shows listed users.
-Otherwise graph shows users that have been using GPUs only.
+  env.gpuusers root user1 user2
 
 =head1 AUTHOR