qemu-img: Fix documentation of convert

It got lost in commit a8d16f9c "qemu-img: Update documentation for -U".

Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 20170515103551.31313-1-famz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>