fixed an incorrect variable name

discovered when working on https://github.com/GigaOM/legacy-pro/issues/3828
Merge pull request #4 from wluo/mailchimp-fix

fixed an incorrect variable name
Merge branch 'master' of github.com:GigaOM/go-mailchimp into github-3828

Conflicts:
	components/class-go-mailchimp-api.php
moved admin-related functions into its own class/file