[Key Vault Admin] Fixed bad reference in the readme (#11599)

The readme had a bad reference in one of the samples.

@zzhxiaofeng Is this what you meant?

Fixes #11491

sdk/keyvault/keyvault-admin/README.md

@@ -107,7 +107,7 @@ Once you've populated the **AZURE_CLIENT_ID**, **AZURE_CLIENT_SECRET** and **AZU
 
 ```ts
 import { DefaultAzureCredential } from "@azure/identity";
-import { KeyVaultBackupClient } from "@azure/keyvault-admin";
+import { KeyVaultAccessControlClient } from "@azure/keyvault-admin";
 
 const credentials = new DefaultAzureCredential();