fix typos in vtbackup.go
fix typos in vtbackup.go

Signed-off-by: 常仲民@daocloud <95e483674d6aaddca68d52cac2554f8971532662@daocloud.io>

vitessio/vitess,mahak/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,tinyspeck/vitess,tinyspeck/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess,mahak/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,mahak/vitess,tinyspeck/vitess,vitessio/vitess