Fix wrong identifier name in Documentation/kref.txt

There's a typo / wrong identifier name in Documentation/kref.txt. Fix it.
Signed-off-by: Satyam Sharma <ssatyam@cse.iitk.ac.in>
Acked-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

Documentation/kref.txt

@@ -67,7 +67,7 @@ void more_data_handling(void *cb_data)
 	.
 	. do stuff with data here
 	.
-	kref_put(data, data_release);
+	kref_put(&data->refcount, data_release);
 }
 
 int my_data_handler(void)