fix a tiny bug about tikv-ctl mvcc subcommand (#4910)
fix a tiny bug about tikv-ctl mvcc subcommand (#4910)

Signed-off-by: qupeng <61d746964f2cf9aa900605464d8422efaad5ea7d@pingcap.com>
pingcap/tikv,BusyJay/tikv,BusyJay/tikv,pingcap/tikv,BusyJay/tikv,c4pt0r/tikv,BusyJay/tikv