fix in readme file for latest sdk generation (#15037)

Author: asmaskar

specification/recoveryservicesbackup/resource-manager/readme.md

@@ -28,7 +28,7 @@ These are the global settings for the RecoveryServicesBackup API.
 title: Recovery Services Backup Client
 description: Open API 2.0 Specs for Azure RecoveryServices Backup service
 openapi-type: arm
-tag: package-2021-01
+tag: package-2021-04
 csharp-sdks-folder: ./Generated/CSharp
 python-sdks-folder: ./Generated/Python
 go-sdk-folder: ./Generated/Golang