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>