op-proposer: Cleanup flag usage string (#9680)

op-proposer/flags/flags.go

@@ -68,7 +68,7 @@ var (
 	}
 	ActiveSequencerCheckDurationFlag = &cli.DurationFlag{
 		Name:    "active-sequencer-check-duration",
-		Usage:   "The duration between checks to determine the active sequencer endpoint. ",
+		Usage:   "The duration between checks to determine the active sequencer endpoint.",
 		Value:   2 * time.Minute,
 		EnvVars: prefixEnvVars("ACTIVE_SEQUENCER_CHECK_DURATION"),
 	}