Fix switch script path in README.md

Author: Piet Schrijver

README.md

@@ -83,7 +83,7 @@ make testacc_cleanup
 The Azure identity role test can only run against an actual Azure postgres instance, also only a Microsoft Entra administrator is allowed to manage Microsoft Entra roles
 In order to run the Azure identity test, run the following commands:
 ```sh
-source tests/
+source tests/switch_azure_ad_user.sh
 
 TF_LOG=INFO go test -v ./postgresql -run ^TestAccPostgresqlRole_AzureIdentity
 ```