block: bio: fix a warning at the kernel-doc markups

Using "@bio's parent" causes the following waring:
	./block/bio.c:10: WARNING: Inline emphasis start-string without end-string.

The main problem here is that this would be converted into:

	**bio**'s parent

By kernel-doc, which is not a valid notation. It would be
possible to use, instead, this kernel-doc markup:

	``bio's`` parent

Yet, here, is probably simpler to just use an altenative language:

	the parent of @bio

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