Removed the string "REST"

This list requires Azure service name and does not need string "REST".
In particular, Speech Services has REST API feature to use when the Speech SDK is not available.
Extra string "REST" leads to the misleading that access tokens are not available with Speech SDK.

Author: isnullnull

articles/cognitive-services/authentication.md

@@ -123,8 +123,8 @@ curl -X POST 'https://api.cognitive.microsofttranslator.com/translate?api-versio
 Some Azure Cognitive Services accept, and in some cases require, an access token. Currently, these services support access tokens:
 
 * Text Translation API
-* Speech Services: Speech-to-text REST API
-* Speech Services: Text-to-speech REST API
+* Speech Services: Speech-to-text API
+* Speech Services: Text-to-speech API
 
 >[!NOTE]
 > QnA Maker also uses the Authorization header, but requires an endpoint key. For more information, see [QnA Maker: Get answer from knowledge base](./qnamaker/quickstarts/get-answer-from-knowledge-base-using-url-tool.md).