Update strings.xml

Author: bhansen3

Speech/TextToSpeech/Android/Sample/app/src/main/res/values/strings.xml

@@ -1,5 +1,5 @@
 <resources>
-    <!-- Please refer to http://oxford-portal.azurewebsites.net/doc/general/subscription-key-mgmt to get your subscription key -->
+    <!-- Please refer to https://www.projectoxford.ai/ to get your subscription key -->
     <!-- If you have one subscription key, you can add it here to use the service -->
     <string name="subscription_key">Please_add_the_subscription_key_here</string>