mm/mmap: fix typo in comment

Replace "parital" with "partial".

Link: https://lkml.kernel.org/r/20230110145353.1658435-1-vernon2gm@gmail.com


Signed-off-by: Vernon Yang <vernon2gm@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>