Fix javadoc copied from CellStyleGenerator

Change-Id: I305c0aaeba04c4c1416ecebd708f377f8f28f4a1

server/src/com/vaadin/ui/Tree.java

@@ -1413,7 +1413,7 @@ public class Tree extends AbstractSelect implements Container.Hierarchical,
 
     /**
      * ItemStyleGenerator can be used to add custom styles to tree items. The
-     * CSS class name that will be added to the cell content is
+     * CSS class name that will be added to the item content is
      * <tt>v-tree-node-[style name]</tt>.
      */
     public interface ItemStyleGenerator extends Serializable {