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

Documentation/x86/x86_64/mm.rst

 .. SPDX-License-Identifier: GPL-2.0
 
-================
-Memory Managment
-================
+=================
+Memory Management
+=================
 
 Complete virtual memory map with 4-level page tables
 ====================================================