Fix references to nommu-mmap.rst

nommu-mmap.rst was moved to Documentation/admin-guide/mm; this patch
updates the remaining stale references to Documentation/mm.

Fixes: 800c02f5d030 ("docs: move nommu-mmap.txt to admin-guide and rename to ReST")
Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20200812092230.27541-1-steve@sk2.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/mm/nommu.c b/mm/nommu.c
index 75a3271..0df7ca3 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -5,7 +5,7 @@
  *  Replacement code for mm functions to support CPU's that don't
  *  have any form of memory management unit (thus no virtual memory).
  *
- *  See Documentation/mm/nommu-mmap.rst
+ *  See Documentation/admin-guide/mm/nommu-mmap.rst
  *
  *  Copyright (c) 2004-2008 David Howells <dhowells@redhat.com>
  *  Copyright (c) 2000-2003 David McCullough <davidm@snapgear.com>