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>

Author: Baolin Wang

Documentation/admin-guide/mm/transhuge.rst

@@ -436,7 +436,7 @@ AnonHugePmdMapped).
 The number of file transparent huge pages mapped to userspace is available
 by reading ShmemPmdMapped and ShmemHugePages fields in ``/proc/meminfo``.
 To identify what applications are mapping file transparent huge pages, it
-is necessary to read ``/proc/PID/smaps`` and count the FileHugeMapped fields
+is necessary to read ``/proc/PID/smaps`` and count the FilePmdMapped fields
 for each mapping.
 
 Note that reading the smaps file is expensive and reading it