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

Documentation/block/biovecs.rst

@@ -47,7 +47,7 @@ Having a real iterator, and making biovecs immutable, has a number of
 advantages:
 
  * Before, iterating over bios was very awkward when you weren't processing
-   exactly one bvec at a time - for example, bio_copy_data() in fs/bio.c,
+   exactly one bvec at a time - for example, bio_copy_data() in block/bio.c,
    which copies the contents of one bio into another. Because the biovecs
    wouldn't necessarily be the same size, the old code was tricky convoluted -
    it had to walk two different bios at the same time, keeping both bi_idx and