chore(Natural Language Understanding): Apply manual changes

Author: lpatino10

natural-language-understanding/src/main/java/com/ibm/watson/natural_language_understanding/v1/NaturalLanguageUnderstanding.java

@@ -45,7 +45,7 @@
  */
 public class NaturalLanguageUnderstanding extends BaseService {
 
-  private static final String DEFAULT_SERVICE_NAME = "natural-language-understanding";
+  private static final String DEFAULT_SERVICE_NAME = "natural_language_understanding";
 
   private static final String DEFAULT_SERVICE_URL
       = "https://gateway.watsonplatform.net/natural-language-understanding/api";