unused-for is a deprecated synonym

See https://github.com/moby/moby/blob/9e7d5ac5ea55dc57185f49476e03d6efa76b3512/builder/builder-next/builder.go#L574

Signed-off-by: Samuel Cochran <sj26@sj26.com>

cli/command/builder/prune.go

@@ -45,7 +45,7 @@ func NewPruneCommand(dockerCli command.Cli) *cobra.Command {
 	flags := cmd.Flags()
 	flags.BoolVarP(&options.force, "force", "f", false, "Do not prompt for confirmation")
 	flags.BoolVarP(&options.all, "all", "a", false, "Remove all unused images, not just dangling ones")
-	flags.Var(&options.filter, "filter", "Provide filter values (e.g. 'unused-for=24h')")
+	flags.Var(&options.filter, "filter", "Provide filter values (e.g. 'until=24h')")
 	flags.Var(&options.keepStorage, "keep-storage", "Amount of disk space to keep for cache")
 
 	return cmd