added "options" parameter to the signature of validateAppId

[zsmuller]

In the current Parse-Server guide the description of "Custom authentication" contains this:

On this module, you need to implement and export those two functions validateAuthData(authData, options) {} and validateAppId(appIds, authData) {}.

However the validateAppId() function is handed a third "options" parameter as well (just as validateAuthData()), so the correct sentence sounds like this:

On this module, you need to implement and export those two functions validateAuthData(authData, options) {} and validateAppId(appIds, authData, options) {}.

This PR contains only this small change.

[flovilmart]

Thanks!