Add runc list man change

Follow up https://github.com/opencontainers/runc/pull/939

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

list.go

@@ -45,11 +45,11 @@ Where the given root is specified via the global option "--root"
 
 EXAMPLE 1:
 To list containers created via the default "--root":
-       runc list
+       # runc list
 
 EXAMPLE 2:
 To list containers created using a non-default value for "--root":
-       runc --root value list`,
+       # runc --root value list`,
 	Flags: []cli.Flag{
 		cli.StringFlag{
 			Name:  "format, f",

man/runc-list.8.md

@@ -2,7 +2,17 @@
    runc list - lists containers started by runc with the given root
 
 # SYNOPSIS
-   runc list [command options] [arguments...]
+   runc list [command options]
+
+# EXAMPLE
+Where the given root is specified via the global option "--root"
+(default: "/run/runc").
+
+To list containers created via the default "--root":
+       # runc list
+
+To list containers created using a non-default value for "--root":
+       # runc --root value list
 
 # OPTIONS
    --format value, -f value     select one of: table or json (default: "table")