[AppGW] Start-AzApplicationGateway docs example (Azure#20699)

* [AppGW] Start-AzApplicationGateway docs example

Adding Get-AzApplicationGateway command to be consistent with https://github.com/Azure/azure-powershell/blob/main/src/Network/Network/help/Start-AzApplicationGateway.md

* Update Start-AzApplicationGateway.md

Consistency with https://github.com/Azure/azure-powershell/blob/main/src/Network/Network/help/Stop-AzApplicationGateway.md

Author: JackStromberg

src/Network/Network/help/Start-AzApplicationGateway.md

@@ -25,7 +25,8 @@ The **Start-AzApplicationGateway** cmdlet starts an Azure application gateway
 
 ### Example1: Start an application gateway
 ```powershell
-$AppGw = Start-AzApplicationGateway -ApplicationGateway $AppGw
+$AppGw = Get-AzApplicationGateway -Name Test -ResourceGroupName Appgwtest
+Start-AzApplicationGateway -ApplicationGateway $AppGw
 ```
 
 This command starts the application gateway stored in the $AppGw variable.