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.comSigned-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>

include/linux/highmem.h

@@ -86,8 +86,8 @@ static inline void kmap_flush_unused(void);
  * virtual address of the direct mapping. Only real highmem pages are
  * temporarily mapped.
  *
- * While it is significantly faster than kmap() for the higmem case it
- * comes with restrictions about the pointer validity.
+ * While kmap_local_page() is significantly faster than kmap() for the highmem
+ * case it comes with restrictions about the pointer validity.
  *
  * On HIGHMEM enabled systems mapping a highmem page has the side effect of
  * disabling migration in order to keep the virtual address stable across