Update the ref guide to reflect the vlan brief output (sonic-net#2731)

What I did
show vlan brief will only be showing dhcpv4 addresses and not dhcpv6 destination

Signed-off-by: Vivek Reddy Karri <vkarri@nvidia.com>

doc/Command-Reference.md

@@ -9793,7 +9793,7 @@ Go Back To [Beginning of the document](#) or [Beginning of this section](#System
 
 **show vlan brief**
 
-This command displays brief information about all the vlans configured in the device. It displays the vlan ID, IP address (if configured for the vlan), list of vlan member ports, whether the port is tagged or in untagged mode, the DHCP Helper Address, and the proxy ARP status
+This command displays brief information about all the vlans configured in the device. It displays the vlan ID, IP address (if configured for the vlan), list of vlan member ports, whether the port is tagged or in untagged mode, the DHCPv4 Helper Address, and the proxy ARP status
 
 - Usage:
   ```