mm/page-writeback: fix a typo in comment "effictive"->"effective"

There is a typo in comment, fix it.
Signed-off-by: Ethon Paul <ethp@qq.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200411003513.14613-1-ethp@qq.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page-writeback.c

@@ -257,7 +257,7 @@ static void wb_min_max_ratio(struct bdi_writeback *wb,
  * requiring writeback.
  *
  * This number of dirtyable pages is the base value of which the
- * user-configurable dirty ratio is the effictive number of pages that
+ * user-configurable dirty ratio is the effective number of pages that
  * are allowed to be actually dirtied.  Per individual zone, or
  * globally by using the sum of dirtyable pages over all zones.
  *