fix: Use a more reliable WHOIS server for .mx

[AlfredoRamos]

Summary

In #20 it added support for .MX domains, however that uses the whois.mx server, which sometimes, and for some ccSLD like .org.mx, gives time-out errors.

$ go build -o bin/whois cmd/whois/main.go
$ bin/whois [REDACTED].org.mx
dial tcp 5.161.210.2:43: i/o timeout

Replacing it with whois.nic.mx fixes this issue for all *.mx extensions.

Checklist

• [x] Tested and/or added tests to validate that the changes work as intended, if applicable.