docs: block/biovecs: update the location of bio.c

Replace fs with block since bio.c had been moved to block folder.

Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Link: https://lore.kernel.org/r/20200106103735.10327-1-guoqing.jiang@cloud.ionos.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>