docs/interop/qcow2.txt: fix description about "zlib" clusters

"zlib" clusters are actually raw deflate (RFC1951) clusters without
zlib headers.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
Message-Id: <168424874322.11954.1340942046351859521-0@git.sr.ht>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
1 file changed