storage: Fix /var partition re-use example

Fixes:

```
Invalid Butane config at ./modules/ROOT/pages/storage.adoc:92:
  warning at $.storage.disks.0.partitions.0.number, line 8 col 7: partitions cannot be reused by label; number must be specified except on boot disk (/dev/disk/by-id/coreos-boot-disk) or when wipe_table is true
  Config produced warnings and --strict was specified
```

See: coreos/butane#711 (comment)

Author: travier

modules/ROOT/pages/storage.adoc

@@ -98,6 +98,7 @@ storage:
     partitions:
     - size_mib: 0
       start_mib: 0
+      number: 1
       label: log
   filesystems:
     - path: /var/log