Documentation: x86: fix a missing word in x86_64/mm.rst.

This patch adds a missing word in x86/x86_64/mm.rst, without which
the note reads awkwardly.

Signed-off-by: Wei Lin Chang <r09922117@csie.ntu.edu.tw>
Link: https://lore.kernel.org/r/20201015062242.26296-1-r09922117@csie.ntu.edu.tw


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