Update opt-out-api-quickstart.md

Author: dbasantes

articles/communication-services/quickstarts/sms/opt-out-api-quickstart.md

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+[!INCLUDE [Public Preview Disclaimer](../../includes/public-preview-include.md)]
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 # Send OptOut API requests with API (HMAC)
 
 This article describes how to enable opt-out management for your Azure Communication Services resource using hash message authentication code (HMAC) based authentication.