mm/mmap: fix comment of unmapped_area{_topdown}

mm/mmap: fix comment of unmapped_area{_topdown}

The low_limit of unmapped area information is inclusive, and the
hight_limit is not, so make symbol to be [ instead of (.

And replace hight_limit to high_limit.

Link: https://lkml.kernel.org/r/20230111132036.801404-1-vernon2gm@gmail.com
Fixes: 3499a13168da ("mm/mmap: use maple tree for unmapped_area{_topdown}")
Signed-off-by: Vernon Yang <vernon2gm@gmail.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>