manual: add manual for ps command

This patch add manual for ps command, and remove remove example
for --format command, which is not necessary.

Suggested-by: Aleksa Sarai <asarai@suse.de>
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>

man/runc-ps.8.md

@@ -0,0 +1,13 @@
+# NAME
+   runc ps - ps displays the processes running inside a container
+
+# SYNOPSIS
+   runc ps [command options] <container-id> <ps options>
+
+# OPTIONS
+   --format, -f         select one of: table(default) or json
+
+The default format is table.  The following will output the processes of a container
+in json format:
+
+    # runc ps -f json

ps.go

@@ -21,12 +21,7 @@ var psCommand = cli.Command{
 		cli.StringFlag{
 			Name:  "format, f",
 			Value: "",
-			Usage: `select one of: ` + formatOptions + `.
-
-The default format is table.  The following will output the processes of a container
-in json format:
-
-    # runc ps -f json`,
+			Usage: `select one of: ` + formatOptions,
 		},
 	},
 	Action: func(context *cli.Context) error {