mm: hugetlb_vmemmap: fix reference to nonexistent file

The directory this file is in was renamed but the reference didn't get
updated.  Fix it.

Link: https://lkml.kernel.org/r/20231022185619.919397-1-vegard.nossum@oracle.com
Fixes: ee65728e

 ("docs: rename Documentation/vm to Documentation/mm")
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Rik van Riel <riel@surriel.com>
Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Wu XiangCheng <bobwxc@email.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>