Misleading error message about license key

Thanks for fullPage.js!

See #3978 where I thought a license key was mandatory, and that that was why my page wasn't working.

The documentation for the `licenseKey` option at https://alvarotrigo.com/fullPage/docs/#licensekey says:

"This option is compulsory."

When my page didn't work, and I saw the browser console message:

```
fullPage: Incorrect `licenseKey`. Get one for fullPage.js version 4 here: [fullpage.min.js:11:2133](https://cdnjs.cloudflare.com/ajax/libs/fullPage.js/4.0.31/fullpage.min.js)
fullPage: https://alvarotrigo.com/fullPage/pricing
```

and then read the above documentation, I concluded that I needed a license key for the code to work.

Since this is not the case, it would be nice if the console log message and documentation made clear that having a license key is a legal requirement for certain uses of the code rather than a technical requirement for any use of the code.