Update XMLEditorDemo.java (#1122)

Accommodate multi-line comments and also allow tags in comments.

Author: Jugen

richtextfx-demos/src/main/java/org/fxmisc/richtext/demo/XMLEditorDemo.java

@@ -19,7 +19,7 @@
 public class XMLEditorDemo extends Application {
 
     private static final Pattern XML_TAG = Pattern.compile("(?<ELEMENT>(</?\\h*)(\\w+)([^<>]*)(\\h*/?>))"
-    		+"|(?<COMMENT><!--[^<>]+-->)");
+    		+"|(?<COMMENT><!--(.|\\v)+?-->)");
 
     private static final Pattern ATTRIBUTES = Pattern.compile("(\\w+\\h*)(=)(\\h*\"[^\"]+\")");