Documentation should note to use 2-digit letter ISO country code, not numeric

[dustinbolton]

The documentation does not specify the country code as needing to the 2-digit letter ISO country code (eg "US") and not the numeric country code. It would be great to update the docs to reflect this.

A listing of the appropriate letter-based ISO country codes can be found here:
https://countrycode.org/

This should help others where I got confused. Thanks!