Update Set-AzApplicationGatewayBackendAddressPool.md (#21458)

Updating with Set-AzApplicationGateway command without which the application gateway configuration changes are not saved.

Author: gitanjali1993

src/Network/Network/help/Set-AzApplicationGatewayBackendAddressPool.md

@@ -29,6 +29,7 @@ Back-end addresses can be specified as IP addresses, fully-qualified domain name
 ```powershell
 $AppGw = Get-AzApplicationGateway -Name "ApplicationGateway01" -ResourceGroupName "ResourceGroup01"
 $AppGw = Set-AzApplicationGatewayBackendAddressPool -ApplicationGateway $AppGw -Name "Pool02" -BackendFqdns "contoso1.com", "contoso2.com"
+Set-AzApplicationGateway -ApplicationGateway $AppGw
 ```
 
 The first command gets the application gateway named ApplicationGateway01 in the resource group named ResourceGroup01, and stores it in the $AppGw variable.
@@ -38,6 +39,7 @@ The second command updates the back-end address pool of the application gateway
 ```powershell
 $AppGw = Get-AzApplicationGateway -Name "ApplicationGateway01" -ResourceGroupName "ResourceGroup01"
 $AppGw = Set-AzApplicationGatewayBackendAddressPool -ApplicationGateway $AppGw -Name "Pool02" -BackendIPAddresses "10.10.10.10", "10.10.10.11"
+Set-AzApplicationGateway -ApplicationGateway $AppGw
 ```
 
 The first command gets the application gateway named ApplicationGateway01 in the resource group named ResourceGroup01, and stores it in the $AppGw variable.