Fix deprecation warning

Original class:
https://github.com/jhy/jsoup/blob/9e36f9e/src/main/java/org/jsoup/examples/HtmlToPlainText.java

core/src/main/java/de/danoeh/antennapod/core/util/syndication/HtmlToPlainText.java

@@ -33,8 +33,8 @@ public class HtmlToPlainText {
      */
     public String getPlainText(Element element) {
         FormattingVisitor formatter = new FormattingVisitor();
-        NodeTraversor traversor = new NodeTraversor(formatter);
+        // walk the DOM, and call .head() and .tail() for each node
-        traversor.traverse(element); // walk the DOM, and call .head() and .tail() for each node
+        NodeTraversor.traverse(formatter, element);
 
         return formatter.toString();
     }