Make DukeParsingException package-private

Since it's only used in the parser package

src/main/java/duke/parser/DukeParsingException.java

@@ -3,8 +3,8 @@
 /**
  * This exception is handled within the route function in the Duke class.
  */
-public class DukeParsingException extends Exception {
-    public DukeParsingException(String message) {
+class DukeParsingException extends Exception {
+    DukeParsingException(String message) {
         super(message);
     }
 }