bio: fix kerneldoc documentation for bio_alloc_kiocb()

Apparently the last fixup got butter fingered a bit, the correct variable
name is 'nr_vecs', not 'nr_iovecs'.

Link: https://lore.kernel.org/lkml/20210903164939.02f6e8c5@canb.auug.org.au/Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -1688,7 +1688,7 @@ EXPORT_SYMBOL(bioset_init_from_src);
 /**
  * bio_alloc_kiocb - Allocate a bio from bio_set based on kiocb
  * @kiocb:	kiocb describing the IO
- * @nr_iovecs:	number of iovecs to pre-allocate
+ * @nr_vecs:	number of iovecs to pre-allocate
  * @bs:		bio_set to allocate from
  *
  * Description: