chore: updating token endpoint to iam domain

Author: manisha1997

examples/BearerTokenAuthentication.md

@@ -3,6 +3,7 @@ using System;
 using Twilio;
 using Twilio.Base;
 using Twilio.Exceptions;
+using Twilio.Credential;
 using Twilio.Rest.Api.V2010.Account;
 using Twilio.Rest.PreviewIam.Organizations;
 

src/Twilio/Http/BearerToken/ApiTokenManager.cs

@@ -1,4 +1,4 @@
-using Twilio.Rest.PreviewIam.V1;
+using Twilio.Rest.Iam.V1;
 using Twilio.Exceptions;
 using System;
 using Twilio.Annotations;
@@ -64,4 +64,4 @@ public string fetchAccessToken(){
            return token.AccessToken;
         }
     }
-}
+}

src/Twilio/Http/BearerToken/OrgsTokenManager.cs

@@ -1,4 +1,4 @@
-using Twilio.Rest.PreviewIam.V1;
+using Twilio.Rest.Iam.V1;
 using Twilio.Exceptions;
 using System;
 using Twilio.Annotations;
@@ -63,4 +63,4 @@ public string fetchAccessToken(){
            return token.AccessToken;
         }
     }
-}
+}