Fix usage string for vat
Fix usage string for vat

Change-Id: Idad65cbb3765500a66f1097126076a2c5fdb4f1b
Signed-off-by: Jerome Tollet <7691d528ed5187b80d535c68197d9578c8bc33cd@cisco.com>

chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp