Update README example

Updates example with accept header to match our docs.

Author: brettmortensen

openapi/templates/README.mustache

@@ -59,8 +59,10 @@ import java.util.*;
 
 public class Example {
     public static void main(String[] args) {
-        // Configure environment. https://int-api.mx.com for development, https://api.mx.com for production
         ApiClient defaultClient = Configuration.getDefaultApiClient();
+        defaultClient.addDefaultHeader("Accept", "application/vnd.mx.api.v1+json");
+
+        // Configure environment. https://int-api.mx.com for development, https://api.mx.com for production
         defaultClient.setBasePath("https://int-api.mx.com");
 
         // Configure with your Client ID/API Key from https://dashboard.mx.com