assoc_array: fix path to assoc_array documentation

Signed-off-by: NAlexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

lib/assoc_array.c

 /* Generic associative array implementation.
  *
- * See Documentation/assoc_array.txt for information.
+ * See Documentation/core-api/assoc_array.rst for information.
  *
  * Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
  * Written by David Howells (dhowells@redhat.com)