docs/mm: Physical Memory: correct spelling in reference to CONFIG_PAGE_EXTENSION

docs/mm: Physical Memory: correct spelling in reference to CONFIG_PAGE_EXTENSION

Commit 5d8c5e430a63 ("docs/mm: Physical Memory: add structure, introduction
and nodes description") slips in a minor spelling mistake for the config
PAGE_EXTENSION.

Correct the config name in the physical-memory documentation.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Link: https://lore.kernel.org/r/20230215100808.9613-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>