docbook: fix bio missing parameter

Fix fs/bio.c kernel-doc parameter warning:
Warning(linux-2.6.25-git14//fs/bio.c:972): No description found for parameter 'reading'
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

fs/bio.c

@@ -963,6 +963,7 @@ static void bio_copy_kern_endio(struct bio *bio, int err)
  *	@data: pointer to buffer to copy
  *	@len: length in bytes
  *	@gfp_mask: allocation flags for bio and page allocation
+ *	@reading: data direction is READ
  *
  *	copy the kernel address into a bio suitable for io to a block
  *	device. Returns an error pointer in case of error.