Fix the typo in ps

Signed-off-by: Kai Qiang Wu(Kennan) <wkqwu@cn.ibm.com>

docs/reference/commandline/port.md

@@ -20,7 +20,7 @@ parent = "smn_cli"
 You can find out all the ports mapped by not specifying a `PRIVATE_PORT`, or
 just a specific mapping:
 
-    $ docker ps test
+    $ docker ps
     CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS                                            NAMES
     b650456536c7        busybox:latest      top                 54 minutes ago      Up 54 minutes       0.0.0.0:1234->9876/tcp, 0.0.0.0:4321->7890/tcp   test
     $ docker port test