fw_update url paths on 8.0 return 404

Hi all,

Just a small niggle post-8.0 on -current:

```
Fetching updated firmware.
fw_update: failed.
Cannot fetch http://firmware.openbsd.org/firmware/8.0//SHA256.sig (404
Not Found)
```

http://firmware.openbsd.org/firmware/8.0/ is not there yet; plus the
double trailing slash also might need correcting, I don't recall it
being there in the past.

Yours

rqm