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/20170809185816.11244-1-sj38.park@gmail.com
Fixes: 467c996c

 ("Print out statistics in relation to fragmentation avoidance to /proc/pagetypeinfo")
Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>