minor usage formatting update
minor usage formatting update

Jinglei-Cheng-THU/VASim,Jinglei-Cheng-THU/VASim,Jinglei-Cheng-THU/VASim,jackwadden/VASim,jackwadden/VASim,Jinglei-Cheng-THU/VASim,jackwadden/VASim,jackwadden/VASim