add default value

Author: dihan0604

src/ServiceManagement/ExpressRoute/Commands.ExpressRoute/BgpPeerings/NewAzureBGPPeering.cs

@@ -36,6 +36,7 @@ public class NewAzureBGPPeeringCommand : ExpressRouteBaseCmdlet
         public UInt32 PeerAsn { get; set; }
 
         [Parameter(Mandatory = false, ValueFromPipelineByPropertyName = true, HelpMessage = "Legacy Mode")]
+        [DefaultValue(LegacyMode.False)]
         public LegacyMode LegacyMode { get; set; }
 
         [Parameter(Mandatory = true, ValueFromPipelineByPropertyName = true, HelpMessage = "Primary Peer Subnet")]