Make it clear which flag is missing in the error msg.

Signed-off-by: Jacques Grove <aquarapid@gmail.com>

go/cmd/vttablet/vttablet.go

@@ -61,7 +61,7 @@ func main() {
  servenv.Init()
 
  if *tabletPath == "" {
- log.Exit("tabletPath required")
+ log.Exit("-tablet-path required")
  }
  tabletAlias, err := topoproto.ParseTabletAlias(*tabletPath)
  if err != nil {