Fix typo

Author: akharit

tests/README.md

@@ -32,6 +32,6 @@ environment variables should be defined:
 * `azure_subscription_id`
 * `azure_resource_group_name`
 * `azure_service_principal`
-* `azure_service_princip_secret`
+* `azure_service_principal_secret`
 
 Optionally, you may need to define `azure_tenant_id` or `azure_data_lake_store_url_suffix`.