Missing French translation for "Unknown command: %s"

New translation to be added due to #1540:
```json
"Unknown command: %s": {
    "one": "Commande inconnue : %s",
    "other": "Commandes inconnues : %s"
  },
```

_Originally posted by @mleguen in https://github.com/yargs/yargs/pull/1557_