mm: fix spelling mistake in highmem.h

Substitute "higmem" with "highmem" in highmem.h.

Link: https://lkml.kernel.org/r/20230105121305.30714-1-fmdefrancesco@gmail.com


Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Suggested-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>