trivial: rbtree.txt: fix rb_entry() parameters in sample code

Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/rbtree.txt

@@ -188,5 +188,5 @@ Example:
 
   struct rb_node *node;
   for (node = rb_first(&mytree); node; node = rb_next(node))
-  	printk("key=%s\n", rb_entry(node, int, keystring));
+	printk("key=%s\n", rb_entry(node, struct mytype, node)->keystring);