Changed robots.txt to use disallow instead of allow

While it works with major crawlers like Google, the `Allow:` field isn't a standard that's supported by all bots. For a generic `robots.txt` file like this, the "standard" empty `Disallow:` should probably be used to be compatible with the most bots. Additionally, the existing `robots.txt` file fails validators as it doesn't contain a `Disallow:` field.