Adding API ref link

Author: erhopf

articles/cognitive-services/Speech-Service/index.yml

@@ -123,6 +123,9 @@ conceptualContent:
         - itemType: how-to-guide
           text: About Speaker Recognition
           url: ../Speaker-recognition/Home.md
+        - itemType: how-to-guide
+          text: Speaker Recognition API reference
+          url: https://westus.dev.cognitive.microsoft.com/docs/services/563309b6778daf02acc0a508/operations/5645c3271984551c84ec6797
     - title: Tools
       links:
         - itemType: overview