Fix incomplete sentence in validation msg

Author: dagnir

codegen/src/main/java/software/amazon/awssdk/codegen/naming/DefaultNamingStrategy.java

@@ -507,7 +507,8 @@ private void validateCustomerVisibleName(String name, String location) {
                     ValidationErrorSeverity.DANGER,
                     String.format(
                         "Encountered a name or identifier that the customer will see (%s in the %s) with an underscore. "
-                        + "This isn't idiomatic in Java. Please either remove the underscores",
+                        + "This isn't idiomatic in Java. Please either remove the underscores or set the "
+                        + "'underscoresInNameBehavior' customization to 'ALLOW'.",
                         name, location, supportedBehaviors)
                 ));
             }