docs: mm/page_owner: fix spelling mistakes

Fix several spelling mistakes in page_owner documentation.

Signed-off-by: Chen Xiao <abigwc@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/1670479443-8484-1-git-send-email-abigwc@gmail.com


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