qcow2: fix typo in documentation for qcow2_get_cluster_offset()

Documentation states the num is measured in clusters, but its
actually measured in sectors
Signed-off-by: NDevin Nakamura <devin122@gmail.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -381,10 +381,10 @@ static int copy_sectors(BlockDriverState *bs, uint64_t start_sect,
  * For a given offset of the disk image, find the cluster offset in
  * qcow2 file. The offset is stored in *cluster_offset.
  *
- * on entry, *num is the number of contiguous clusters we'd like to
+ * on entry, *num is the number of contiguous sectors we'd like to
  * access following offset.
  *
- * on exit, *num is the number of contiguous clusters we can read.
+ * on exit, *num is the number of contiguous sectors we can read.
  *
  * Return 0, if the offset is found
  * Return -errno, otherwise.