[Doc]: Add-AzApplicationGatewaySslCertificate example omits required Set-AzApplicationGateway to persist changes

### Type of issue

Other (describe below)

### Feedback

The current example shows how to add an SSL certificate to an Application Gateway by updating the in-memory object returned by Get-AzApplicationGateway and then calling Add-AzApplicationGatewaySslCertificate. However, it does not demonstrate the final step needed to persist those changes back to the Azure resource.
Without calling Set-AzApplicationGateway -ApplicationGateway $AppGW (or Update-AzApplicationGateway in some patterns), users may assume the change is applied, but the certificate is not actually pushed to the Application Gateway.

```
# Persist the change to Azure (required)
Set-AzApplicationGateway -ApplicationGateway $AppGW
```

### Page URL

https://learn.microsoft.com/en-us/powershell/module/az.network/add-azapplicationgatewaysslcertificate?view=azps-15.1.0&viewFallbackFrom=azps-14.1.0

### Content source URL

https://github.com/Azure/azure-powershell/blob/main/src/Network/Network/help/Add-AzApplicationGatewaySslCertificate.md

### Author

@mikefrobbins

### Document Id

d9014bc0-dbaa-ad6f-bb29-f30fe3c2a3bd

### Platform Id

805a1f56-1b93-78c3-7724-9427da6332a5