docs: fix one issue in qcow2 specs

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

docs/specs/qcow2.txt

diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index 65e6325..87bf785 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -262,7 +262,7 @@
         56 - 61:    Reserved (set to 0)
 
 
-Compressed Clusters Descriptor (x = 62 - (cluster_size - 8)):
+Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):
 
     Bit  0 -  x:    Host cluster offset. This is usually _not_ aligned to a
                     cluster boundary!