mm/vmstat.c: fix wrong comment

mm/vmstat.c: fix wrong comment

Comment for pagetypeinfo_showblockcount() is mistakenly duplicated from
pagetypeinfo_show_free()'s comment.  This commit fixes it.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 467c996c1e19 ("Print out statistics in relation to fragmentation avoidance to /proc/pagetypeinfo")
Signed-off-by: SeongJae Park <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>