chore(assistant): revert default service name

Author: kevinkowa

assistant/src/main/java/com/ibm/watson/assistant/v1/Assistant.java

@@ -109,7 +109,7 @@
  */
 public class Assistant extends BaseService {
 
-  public static final String DEFAULT_SERVICE_NAME = "conversation";
+  public static final String DEFAULT_SERVICE_NAME = "assistant";
 
   public static final String DEFAULT_SERVICE_URL =
       "https://api.us-south.assistant.watson.cloud.ibm.com";

assistant/src/main/java/com/ibm/watson/assistant/v2/Assistant.java

@@ -54,7 +54,7 @@
  */
 public class Assistant extends BaseService {
 
-  public static final String DEFAULT_SERVICE_NAME = "conversation";
+  public static final String DEFAULT_SERVICE_NAME = "assistant";
 
   public static final String DEFAULT_SERVICE_URL =
       "https://api.us-south.assistant.watson.cloud.ibm.com";