mm: fix typo in struct pglist_data code comment

change "stat" to "start".

Link: https://lkml.kernel.org/r/20221207074011.GA151242@cloud
Fixes: c959924b

 ("memory tiering: adjust hot threshold automatically")
Signed-off-by: Wang Yong <yongw.kernel@gmail.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>