dm vdo: fix a minor formatting issue in vdo.rst

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

Author: masatake

Documentation/admin-guide/device-mapper/vdo.rst

@@ -241,6 +241,7 @@ Messages
 All vdo devices accept messages in the form:
 
 ::
+
         dmsetup message <target-name> 0 <message-name> <message-parameters>
 
 The messages are: