Documentation: mm: use `s/higmem/highmem/` fix typo for highmem

Documentation: mm: use `s/higmem/highmem/` fix typo for highmem

We should use highmem replace higmem.

Link: https://lkml.kernel.org/r/20230118025403.1531-1-wangdeming@inspur.com
Signed-off-by: Deming Wang <wangdeming@inspur.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Cc: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>