docs/vm: Fix typo in *harden*

Fixes: df4e817b ("mm: page table check")
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Link: https://lore.kernel.org/r/20220117111338.115455-1-pmenzel@molgen.mpg.de


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