Update README.md (Azure#16790)

It's now preview 9 (not 4)

Author: lilyjma

sdk/servicebus/Azure.Messaging.ServiceBus/README.md

@@ -37,7 +37,7 @@ To quickly create the needed Service Bus resources in Azure and to receive a con
 Install the Azure Service Bus client library for .NET with [NuGet](https://www.nuget.org/):
 
 ```PowerShell
-dotnet add package Azure.Messaging.ServiceBus --version 7.0.0-preview.4
+dotnet add package Azure.Messaging.ServiceBus --version 7.0.0-preview.9
 ```
 
 ### Authenticate the client