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

Commit 5d8c5e43 ("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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/mm/physical_memory.rst

@@ -210,7 +210,7 @@ General
 ``node_page_ext``
   For UMA systems that use FLATMEM memory model the 0's node
   ``node_page_ext`` is array of extensions of struct pages. Available only
-  in the kernels built with ``CONFIG_PAGE_EXTENTION`` enabled.
+  in the kernels built with ``CONFIG_PAGE_EXTENSION`` enabled.
 
 ``node_start_pfn``
   The page frame number of the starting page frame in this node.