docs: bio: fix a kerneldoc markup

Fix this warning:

	./block/bio.c:1098: WARNING: Inline emphasis start-string without end-string.

The thing is that *iter is not a valid markup.

That seems to be a typo:
	*iter -> @iter

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>