Normalize the log for docker subcommand usage,

delete unecessary blank after "[OPTIONS] "
and add a blank to the log between options and signature.

To make the code style consistency.

Signed-off-by: Zen Lin(Zhinan Lin) <linzhinan@huawei.com>

api/client/cli.go

@@ -93,6 +93,9 @@ func (cli *DockerCli) Subcmd(name, signature, description string, exitOnError bo
 	flags := flag.NewFlagSet(name, errorHandling)
 	flags.Usage = func() {
 		options := ""
+		if signature != "" {
+			signature = " " + signature
+		}
 		if flags.FlagCountUndeprecated() > 0 {
 			options = " [OPTIONS]"
 		}