docs: place <app> in args usage [ci skip]

cli/app/app.go

@@ -6,8 +6,9 @@ import (
 
 // AppCommand defines the `abra app` command and ets subcommands
 var AppCommand = &cli.Command{
-	Name:  "app",
-	Usage: "Manage your apps",
+	Name:      "app",
+	Usage:     "Manage your apps",
+	ArgsUsage: "<app>",
 	Description: `
 This command provides all the functionality you need to manage the lifecycle of
 your apps. From initial deployment to day-2 operations (e.g. backup/restore) to