docs: mm: fix the incorrect 'FileHugeMapped' field

The '/proc/PID/smaps' does not have the 'FileHugeMapped' field to count
the file transparent huge pages, instead, the 'FilePmdMapped' field should
be used.  Fix it.

Link: https://lkml.kernel.org/r/d520ce3aba2b03b088be30bece732426a939049a.1734425264.git.baolin.wang@linux.alibaba.com


Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>