app/testpmd: fix help of metering commands

app/testpmd: fix help of metering commands

Helps strings syntax is "command : description", the 'command' part was
missing, updated command help strings.

Fixes: 281eeb8afc55 ("app/testpmd: add commands for metering and policing")
Fixes: 30ffb4e67ee3 ("app/testpmd: add commands traffic metering and policing")
Fixes: e63b50162aa3 ("app/testpmd: clean metering and policing commands")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Jasvinder Singh <jasvinder.singh@intel.com>
Acked-by: Xiaoyun Li <xiaoyun.li@intel.com>