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 <[e-mail]>
Acked-by: Mike Rapoport (IBM) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/mm/physical_memory.rst

···
 ``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.