Doc: x86: Fix a typo in mm.rst

Fix a spelling typo in mm.rst.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Link: https://lore.kernel.org/r/20191226162138.17601-1-standby24x7@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>