changed pattern to work with acronyms in underscore conversion

Author: HugoMario

modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultCodegen.java

@@ -3207,7 +3207,7 @@ private void addParentContainer(CodegenModel m, String name, Property property)
      * @return The underscored version of the word
      */
     public static String underscore(String word) {
-        String firstPattern = "([A-Z]+)([A-Z][a-z])";
+        String firstPattern = "([A-Z]+)([A-Z][a-z][a-z]+)";
         String secondPattern = "([a-z\\d])([A-Z])";
         String replacementPattern = "$1_$2";
         // Replace package separator with slash.