Trivial: Remove unuseful doc of "network list" command.

There is actually no "--dhcp" option in "network list" command.
So remove the doc from .rst file.

Change-Id: I481b260f99be635be4de8e6780206af47cdaa7ce

Author: tang-chen

doc/source/command-objects/network.rst

@@ -74,17 +74,12 @@ List networks
 
     os network list
         [--external]
-        [--dhcp <dhcp-id>]
         [--long]
 
 .. option:: --external
 
     List external networks
 
-.. option:: --dhcp <dhcp-id>
-
-    DHCP agent ID
-
 .. option:: --long
 
     List additional fields in output