docs/qcow2: Correct refcount_block_entries

A refblock entry may have a different size than 16 bits, it may even be
smaller than a byte. Correct the refcount_block_entries calculation
accordingly.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

docs/specs/qcow2.txt

@@ -183,7 +183,7 @@ blocks and are exactly one cluster in size.
 Given a offset into the image file, the refcount of its cluster can be obtained
 as follows:
 
-    refcount_block_entries = (cluster_size / sizeof(uint16_t))
+    refcount_block_entries = (cluster_size * 8 / refcount_bits)
 
     refcount_block_index = (offset / cluster_size) % refcount_block_entries
     refcount_table_index = (offset / cluster_size) / refcount_block_entries