dm log writes: fix incorrect comment about the logged sequence example

dm-log-writes records the sequence of completion, not submission, thus
for the following sequence (W=write, C=complete):

  Wa,Wb,Wc,Cc,Ca,FLUSH,FUAd,Cb,CFLUSH,CFUAd

The logged results in log device should be:
  c,a,b,flush,fua

Fix the comment to give a better example.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-log-writes.c

@@ -40,7 +40,7 @@
  *
  * Would result in the log looking like this:
  *
- * c,a,flush,fuad,b,<other writes>,<next flush>
+ * c,a,b,flush,fuad,<other writes>,<next flush>
  *
  * This is meant to help expose problems where file systems do not properly wait
  * on data being written before invoking a FLUSH.  FUA bypasses cache so once it