dm thin metadata: Fix trivial math error in on-disk format documentation

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-thin-metadata.c

@@ -28,7 +28,7 @@
  *
  * - A hierarchical btree, with 2 levels which effectively maps (thin
  *   dev id, virtual block) -> block_time.  Block time is a 64-bit
- *   field holding the time in the low 24 bits, and block in the top 48
+ *   field holding the time in the low 24 bits, and block in the top 40
  *   bits.
  *
  * BTrees consist solely of btree_nodes, that fill a block.  Some are