Fix access modifier of toTreeString method

Author: pcastornii

patternfx-core/src/main/java/com/techsenger/patternfx/core/AbstractParentComponent.java

@@ -150,7 +150,7 @@ protected ObservableList<ChildComponent<?>> getModifiableChildren() {
     @Override
     protected abstract Mediator createMediator();
 
-    public <T> String toTreeString(TreeIterator<T> iterator, BiConsumer<T, StringBuilder> componentAppender) {
+    private <T> String toTreeString(TreeIterator<T> iterator, BiConsumer<T, StringBuilder> componentAppender) {
         var builder = new StringBuilder();
         var sep = System.lineSeparator();
         while (iterator.hasNext()) {