docs/mm: Physical Memory: Fix grammar

it's -> its
referenced to by -> referenced by

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Link: https://lore.kernel.org/r/20230331165254.207526-1-kim.phillips@amd.com


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