Fix add host device example

The example is not right in parameter, and also
one command is same as first one, it should be typo
before, we should use 'rw' as example for that.

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

docs/reference/commandline/run.md

@@ -422,12 +422,12 @@ flag:
     $ docker run --device=/dev/sda:/dev/xvdc --rm -it ubuntu fdisk  /dev/xvdc
 
     Command (m for help): q
-    $ docker run --device=/dev/sda:/dev/xvdc:ro --rm -it ubuntu fdisk  /dev/xvdc
+    $ docker run --device=/dev/sda:/dev/xvdc:r --rm -it ubuntu fdisk  /dev/xvdc
     You will not be able to write the partition table.
 
     Command (m for help): q
 
-    $ docker run --device=/dev/sda:/dev/xvdc --rm -it ubuntu fdisk  /dev/xvdc
+    $ docker run --device=/dev/sda:/dev/xvdc:rw --rm -it ubuntu fdisk  /dev/xvdc
 
     Command (m for help): q