Correct the value type of the --persistent-keepalive

Prefious:

```
      --persistent-keepalive PersistentKeepalive   WireGuard PersistentKeepalive for the SORACOM Arc server, which will override arc.json#persistentKeepalive value (default 60)
```

Corrected:

```
      --persistent-keepalive int        WireGuard "PersistentKeepalive" for the SORACOM Arc server, which will override arc.json#persistentKeepalive value (default 60)
```

This because of the `pflag` package trick; that package extracts
the back-quoted word from the usage text and guess/use that word as
the type of value.
ref: spf13/pflag#200

This might come from the original golang's flag package:

> can be changed by placing a back-quoted name in the flag's usage string;
> the first such item in the message is taken to be a parameter name to
> show in the message and the back quotes are stripped from the message
> when displayed
> https://pkg.go.dev/flag#PrintDefaults

So this patch replaces the back-quotes with double-quotes to avoid the
unexpected behavior.

Signed-off-by: moznion <[email protected]>

Author: moznion

cmd/up.go

@@ -80,7 +80,7 @@ func upCmd() *cobra.Command {
 	}
 
 	cmd.Flags().IntVar(&mtu, "mtu", soratun.DefaultMTU, "MTU for the interface, which will override arc.json#mtu value")
-	cmd.Flags().IntVar(&persistentKeepalive, "persistent-keepalive", soratun.DefaultPersistentKeepaliveInterval, "WireGuard `PersistentKeepalive` for the SORACOM Arc server, which will override arc.json#persistentKeepalive value")
+	cmd.Flags().IntVar(&persistentKeepalive, "persistent-keepalive", soratun.DefaultPersistentKeepaliveInterval, "WireGuard \"PersistentKeepalive\" for the SORACOM Arc server, which will override arc.json#persistentKeepalive value")
 	cmd.Flags().StringVar(&additionalAllowedIPs, "additional-allowed-ips", "", "Comma separated string of additional WireGuard allowed CIDRs, which will be added to arc.json#additionalAllowedIPs array")
 	cmd.Flags().BoolVar(&readStdin, "read-stdin", false, "read configuration from stdin, ignoring --config setting")