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>

block/bio.c

@@ -1093,7 +1093,7 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
  * released.
  *
  * The function tries, but does not guarantee, to pin as many pages as
- * fit into the bio, or are requested in *iter, whatever is smaller. If
+ * fit into the bio, or are requested in @iter, whatever is smaller. If
  * MM encounters an error pinning the requested pages, it stops. Error
  * is returned only if 0 pages could be pinned.
  */