Update Get-AzServiceBusTopic.md

Corrected optional parameters syntex

Author: deepaknasa

src/ServiceBus/ServiceBus/help/Get-AzServiceBusTopic.md

@@ -13,7 +13,7 @@ Returns a description for the specified Service Bus topic.
 ## SYNTAX
 
 ```
-Get-AzServiceBusTopic [-ResourceGroupName] <String> [-Namespace] <String> [[-Name] <String>]
+Get-AzServiceBusTopic [-ResourceGroupName <String>] [-Namespace <String>] [-Name <String>]
  [-MaxCount <Int32>] [-DefaultProfile <IAzureContextContainer>] [<CommonParameters>]
 ```