Correct spelling of 'lists'

Fixes:
Deprecation Notice: Class Mailchimp_ListInterestCategoryInterests located in ./vendor/ebizmarts/mailchimp-lib/src/Mailchimp/ListsInterestCategoryInterests.php does not comply with psr-0 autoloading standard. It will not autoload anymore in Composer v2.0. in phar:///usr/bin/composer/src/Composer/Autoload/ClassMapGenerator.php:201