Wrong serial in in redump/PlayStation2 SLUS21568

The serial number `SLUS21568` is wrong (missing dash). Correct should be `SLUS-21568`. 

```
game (
	name "Arena Football - Road to Glory (USA)"
	region "USA"
	serial "SLUS21568"
	rom ( name "Arena Football - Road to Glory (USA).iso" size 4642078720 crc 51EABFC8 md5 7E1C0D9CBF00A5FD3009A43B57352C65 sha1 76EE647F2AB9AD84D20B50DD5CC0A80FFA1767FB serial "SLUS21568" )
)
```