Add namespace argument to the job verification help text (#16243)

.changelog/16243.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+cli: `job plan` help text for running the plan now includes the `-namespace` flag
+```

command/job_plan.go

@@ -262,6 +262,10 @@ func (c *JobPlanCommand) Run(args []string) int {
 		runArgs.WriteString(fmt.Sprintf("-var-file=%q ", varFile))
 	}
 
+	if c.namespace != "" {
+		runArgs.WriteString(fmt.Sprintf("-namespace=%q ", c.namespace))
+	}
+
 	exitCode := c.outputPlannedJob(job, resp, diff, verbose)
 	c.Ui.Output(c.Colorize().Color(formatJobModifyIndex(resp.JobModifyIndex, runArgs.String(), path)))
 	return exitCode