Documentation: x86: fix typo in x86_64/mm.rst

change "64Gb" to "64GB".

Signed-off-by: Wang Yong <yongw.kernel@gmail.com>
Link: https://lore.kernel.org/r/20221228100141.382325-1-yongw.kernel@gmail.com


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