fixed annoations breaking headings

src/Parser.php

@@ -310,6 +310,7 @@ class Parser {
 			// then we parse the node content
 			$elm = $this->document->createElement("li");
 			foreach ($this->parseText() as $node)
+			 	if ($node instanceof DOMNode)
 				$elm->appendChild($node);
 
 			// now we check, if the level of the next line is higher than the current level.