updated readme with new preview version (#14527)

Author: alluri02

specification/confluent/resource-manager/readme.md

@@ -28,7 +28,7 @@ These are the global settings for the Confluent API.
 title: ConfluentManagementClient
 openapi-type: arm
 openapi-subtype: rpaas
-tag: package-2020-03-01-preview
+tag: package-2021-03-01-preview
 ```
 
 ### Tag: package-2020-03-01