Fix pylint issue for `azure-ai-vision-face`

[msyyc]

for Azure/azure-sdk-for-python#37044, and log is https://dev.azure.com/azure-sdk/public/_build/results?buildId=4113554&view=logs&j=b70e5e73-bbb6-5567-0939-8415943fadb9&t=5da75d26-b661-5189-46a3-f9dd247b5d85.

Log shows the failure is cause by unsubscriptable-object: