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: 800c02f5 ("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>