Update rest-api.md

Parity w/ other QS prereq

Author: eric-urban

articles/cognitive-services/language-service/language-detection/includes/quickstarts/rest-api.md

@@ -15,6 +15,7 @@ ms.custom: ignite-fall-2021
 
 ## Prerequisites
 
+* Azure subscription - [Create one for free](https://azure.microsoft.com/free/cognitive-services)
 * The current version of [cURL](https://curl.haxx.se/).
 * Once you have your Azure subscription, <a href="https://ms.portal.azure.com/#create/Microsoft.CognitiveServicesTextAnalytics"  title="Create a Language resource"  target="_blank">create a Language resource </a> in the Azure portal to get your key and endpoint. After it deploys, click **Go to resource**.
     * You will need the key and endpoint from the resource you create to connect your application to the API. You'll paste your key and endpoint into the code below later in the quickstart.