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>

Author: kimphillamd

Documentation/mm/physical_memory.rst

@@ -19,7 +19,7 @@ a bank of memory very suitable for DMA near peripheral devices.
 
 Each bank is called a node and the concept is represented under Linux by a
 ``struct pglist_data`` even if the architecture is UMA. This structure is
-always referenced to by it's typedef ``pg_data_t``. ``A pg_data_t`` structure
+always referenced by its typedef ``pg_data_t``. A ``pg_data_t`` structure
 for a particular node can be referenced by ``NODE_DATA(nid)`` macro where
 ``nid`` is the ID of that node.